表创建
表的介绍
image.png
- 表是关系数据库的核心
- 表 = 关系
- 表是记录的集合
- 二维表格模型易于人的理解
- MySQL默认存储引擎都是基于行(记录)存储
- 每行记录都是基于列进行组织的
表是数据的集合
select * from table_name limit 1;
集合是无序的，上面的SQL语句的意思是 从表(集合)中随机选出一条数据，结果是不确定的, 不能简单的认为是取出第一条数据。
select * from table_name order by col_name limit 1;
只有通过order by排序之后取出的数据，才是确定的。
表的创建：官方文档
表创建语法
CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name
(create_definition,...)
[table_options]
[partition_options]
CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name
[(create_definition,...)]
[table_options]
[partition_options]
[IGNORE | REPLACE]
[AS] query_expression
CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name
{ LIKE old_tbl_name | (LIKE old_tbl_name) }
create_definition:
col_name column_definition
| {INDEX|KEY} [index_name] [index_type] (key_part,...)
[index_option] ...
| {FULLTEXT|SPATIAL} [INDEX|KEY] [index_name] (key_part,...)
[index_option] ...
| [CONSTRAINT [symbol]] PRIMARY KEY
[index_type] (key_part,...)
[index_option] ...
| [CONSTRAINT [symbol]] UNIQUE [INDEX|KEY]
[index_name] [index_type] (key_part,...)
[index_option] ...
| [CONSTRAINT [symbol]] FOREIGN KEY
[index_name] (col_name,...)
reference_definition
| CHECK (expr)
column_definition:
data_type [NOT NULL | NULL] [DEFAULT default_value]
[AUTO_INCREMENT] [UNIQUE [KEY]] [[PRIMARY] KEY]
[COMMENT 'string']
[COLLATE collation_name]
[COLUMN_FORMAT {FIXED|DYNAMIC|DEFAULT}]
[STORAGE {DISK|MEMORY}]
[reference_definition]
| data_type
[COLLATE collation_name]
[GENERATED ALWAYS] AS (expr)
[VIRTUAL | STORED] [NOT NULL | NULL]
[UNIQUE [KEY]] [[PRIMARY] KEY]
[COMMENT 'string']
[reference_definition]
data_type:
(see Chapter 11, Data Types)
key_part:
col_name [(length)] [ASC | DESC]
index_type:
USING {BTREE | HASH}
index_option:
KEY_BLOCK_SIZE [=] value
| index_type
| WITH PARSER parser_name
| COMMENT 'string'
reference_definition:
REFERENCES tbl_name (key_part,...)
[MATCH FULL | MATCH PARTIAL | MATCH SIMPLE]
[ON DELETE reference_option]
[ON UPDATE reference_option]
reference_option:
RESTRICT | CASCADE | SET NULL | NO ACTION | SET DEFAULT
table_options:
table_option [[,] table_option] ...
table_option:
AUTO_INCREMENT [=] value
| AVG_ROW_LENGTH [=] value
| [DEFAULT] CHARACTER SET [=] charset_name
| CHECKSUM [=] {0 | 1}
| [DEFAULT] COLLATE [=] collation_name
| COMMENT [=] 'string'
| COMPRESSION [=] {'ZLIB'|'LZ4'|'NONE'}
| CONNECTION [=] 'connect_string'
| {DATA|INDEX} DIRECTORY [=] 'absolute path to directory'
| DELAY_KEY_WRITE [=] {0 | 1}
| ENCRYPTION [=] {'Y' | 'N'}
| ENGINE [=] engine_name
| INSERT_METHOD [=] { NO | FIRST | LAST }
| KEY_BLOCK_SIZE [=] value
| MAX_ROWS [=] value
| MIN_ROWS [=] value
| PACK_KEYS [=] {0 | 1 | DEFAULT}
| PASSWORD [=] 'string'
| ROW_FORMAT [=] {DEFAULT|DYNAMIC|FIXED|COMPRESSED|REDUNDANT|COMPACT}
| STATS_AUTO_RECALC [=] {DEFAULT|0|1}
| STATS_PERSISTENT [=] {DEFAULT|0|1}
| STATS_SAMPLE_PAGES [=] value
| TABLESPACE tablespace_name [STORAGE {DISK|MEMORY}]
| UNION [=] (tbl_name[,tbl_name]...)
partition_options:
PARTITION BY
{ [LINEAR] HASH(expr)
| [LINEAR] KEY [ALGORITHM={1|2}] (column_list)
| RANGE{(expr) | COLUMNS(column_list)}
| LIST{(expr) | COLUMNS(column_list)} }
[PARTITIONS num]
[SUBPARTITION BY
{ [LINEAR] HASH(expr)
| [LINEAR] KEY [ALGORITHM={1|2}] (column_list) }
[SUBPARTITIONS num]
]
[(partition_definition [, partition_definition] ...)]
partition_definition:
PARTITION partition_name
[VALUES
{LESS THAN {(expr | value_list) | MAXVALUE}
|
IN (value_list)}]
[[STORAGE] ENGINE [=] engine_name]
[COMMENT [=] 'string' ]
[DATA DIRECTORY [=] 'data_dir']
[INDEX DIRECTORY [=] 'index_dir']
[MAX_ROWS [=] max_number_of_rows]
[MIN_ROWS [=] min_number_of_rows]
[TABLESPACE [=] tablespace_name]
[(subpartition_definition [, subpartition_definition] ...)]
subpartition_definition:
SUBPARTITION logical_name
[[STORAGE] ENGINE [=] engine_name]
[COMMENT [=] 'string' ]
[DATA DIRECTORY [=] 'data_dir']
[INDEX DIRECTORY [=] 'index_dir']
[MAX_ROWS [=] max_number_of_rows]
[MIN_ROWS [=] min_number_of_rows]
[TABLESPACE [=] tablespace_name]
query_expression:
SELECT ... (Some valid select or union statement)
创建临时表
mysql> select version();
+------------+
| version() |
+------------+
| 5.7.26-log |
+------------+
1 row in set (0.00 sec)
mysql>
mysql> create temporary table a ( a int );
ERROR 1787 (HY000): Statement violates GTID consistency: CREATE TEMPORARY TABLE and DROP TEMPORARY TABLE can only be executed outside transactional context. These statements are also not allowed in a function or trigger because functions and triggers are also considered to be multi-statement transactions.
mysql>
翻译：错误1787（HY000）：语句违反GTID一致性：CREATE TEMPORARY TABLE和DROP TEMPORARY TABLE只能在事务上下文之外执行。函数或触发器中也不允许使用这些语句，因为函数和触发器也被视为多语句事务。
通过查找发现，当autocommit = 1才能创建临时表
# 查看 autocommit
mysql> show variables like 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit | OFF |
+---------------+-------+
1 row in set (0.00 sec)
# 全局修改 autocommit，或者在配置中/etc/my.cnf修改
mysql> set global autocommit = 1;
Query OK, 0 rows affected (0.00 sec)
# 全局修改完成后，需要重新登录MySQL
mysql> show variables like 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit | OFF |
+---------------+-------+
1 row in set (0.00 sec)
mysql> set global autocommit = 1;
Query OK, 0 rows affected (0.00 sec)
mysql> show variables like 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit | OFF |
+---------------+-------+
1 row in set (0.00 sec)
mysql> exit
Bye
[root@iZwz956snfyrvah6yq8sa4Z ~]# mysql -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 14
Server version: 5.7.26-log MySQL Community Server (GPL)
Copyright (c) 2000, 2019, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql> show variables like 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit | ON |
+---------------+-------+
1 row in set (0.00 sec)
# 创建临时表
Database changed
mysql> create temporary table a ( a int );
Query OK, 0 rows affected (0.00 sec)
mysql> insert into a select 123;
Query OK, 1 row affected (0.00 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql> show * from a;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '* from a' at line 1
mysql> select * from a;
+------+
| a |
+------+
| 123 |
+------+
1 row in set (0.00 sec)
另开一个进程
[root@iZwz956snfyrvah6yq8sa4Z ~]# mysql -p
Enter password:
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: YES)
[root@iZwz956snfyrvah6yq8sa4Z ~]# mysql -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 17
Server version: 5.7.26-log MySQL Community Server (GPL)
Copyright (c) 2000, 2019, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql>
mysql>
mysql> use test;
Database changed
mysql>
mysql> show tables;
Empty set (0.00 sec)
mysql>
这时发现，test数据库中是没有a这个表的。因为临时表只有在当前回话内才有效，当MySQL退出时，会话结束后，临时表就释放了。
临时有什么作用呢：可以用来存储一些临时数据或结果。
mysql> create temporary table a ( a int ) ;
Query OK, 0 rows affected (0.00 sec)
mysql> show create table a\G;
*************************** 1. row ***************************
Table: a
Create Table: CREATE TEMPORARY TABLE `a` (
`a` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4
1 row in set (0.00 sec)
ERROR:
No query specified
mysql> show variables like 'default%tmp%';
+----------------------------+--------+
| Variable_name | Value |
+----------------------------+--------+
| default_tmp_storage_engine | InnoDB |
+----------------------------+--------+
1 row in set (0.00 sec)
mysql>
临时表的存储引擎ENGINE是由配置default_tmp_storage_engine值决定的
临时表的存储位置：/usr/local/mysql/data/ibtmp1
mysql> show variables like 'innodb_temp%';
+----------------------------+-----------------------+
| Variable_name | Value |
+----------------------------+-----------------------+
| innodb_temp_data_file_path | ibtmp1:12M:autoextend |
+----------------------------+-----------------------+
1 row in set (0.00 sec)
mysql>
当创建的临时表与已存在的表同名时，查询的结果只能读取到临时的数据
mysql> create table a ( a int, b int );
Query OK, 0 rows affected (0.02 sec)
mysql> insert into a select 1,2;
Query OK, 1 row affected (0.00 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql> select * from a;
+------+------+
| a | b |
+------+------+
| 1 | 2 |
+------+------+
1 row in set (0.00 sec)
mysql> create temporary table a ( a int );
Query OK, 0 rows affected (0.00 sec)
mysql> insert into a select 123;
Query OK, 1 row affected (0.00 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql> select * from a;
+------+
| a |
+------+
| 123 |
+------+
1 row in set (0.00 sec)
mysql>
所以创建临时表时，加上判断条件
mysql> create temporary table if not exists a ( a int );
Query OK, 0 rows affected, 1 warning (0.00 sec)
mysql> show warnings;
+-------+------+--------------------------+
| Level | Code | Message |
+-------+------+--------------------------+
| Note | 1050 | Table 'a' already exists |
+-------+------+--------------------------+
1 row in set (0.00 sec)
mysql>
查看以创建的表结构
mysql> show create table a\G
*************************** 1. row ***************************
Table: a
Create Table: CREATE TEMPORARY TABLE `a` (
`a` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4
1 row in set (0.00 sec)
mysql> desc a;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| a | int(11) | YES | | NULL | |
+-------+---------+------+-----+---------+-------+
1 row in set (0.00 sec)
mysql> show table status like 'a'\G
*************************** 1. row ***************************
Name: a
Engine: InnoDB
Version: 10
Row_format: Dynamic
Rows: 1
Avg_row_length: 8192
Data_length: 8192
Max_data_length: 0
Index_length: 0
Data_free: 0
Auto_increment: NULL
Create_time: 2019-05-30 22:58:01
Update_time: 2019-05-30 22:58:15
Check_time: NULL
Collation: utf8mb4_general_ci
Checksum: NULL
Create_options:
Comment:
1 row in set (0.00 sec)
mysql>
外键约束
[CONSTRAINT [symbol]] FOREIGN KEY
[index_name] (col_name, ...)
REFERENCES tbl_name (col_name,...)
[ON DELETE reference_option]
[ON UPDATE reference_option]
reference_option:
RESTRICT | CASCADE | SET NULL | NO ACTION | SET DEFAULT
外键约束的介绍
CREATE TABLE product ( -- 商品表
category INT NOT NULL, -- 商品种类
id INT NOT NULL, -- 商品id
price DECIMAL,
PRIMARY KEY(category, id) -- 主键是 (category, id)
) ENGINE=INNODB;
CREATE TABLE customer ( -- 客户表
id INT NOT NULL, -- 客户id
PRIMARY KEY (id) -- 主键是 id
) ENGINE=INNODB;
CREATE TABLE product_order ( -- 订单表
no INT NOT NULL AUTO_INCREMENT, -- number，自增长
product_category INT NOT NULL, -- 商品种类
product_id INT NOT NULL, -- 商品id
customer_id INT NOT NULL, -- 客户id
PRIMARY KEY(no), -- 主键是 no
INDEX (product_category, product_id), -- 对 (product_category, product_id) 做索引
INDEX (customer_id), -- 对 customer_id 做索引
FOREIGN KEY (product_category, product_id) -- 两个外键约束
REFERENCES product(category, id) -- 字段 product_category 引用自 product表的category
-- 字段 product_id 引用自 product表的id
ON UPDATE CASCADE ON DELETE RESTRICT, -- 级联跟新 和 严格模式删除
FOREIGN KEY (customer_id)
REFERENCES customer(id)
) ENGINE=INNODB;
外键约束的操作
mysql> create table parent (
-> id int not null,
-> primary key (id)
-> ) engine=innodb;
Query OK, 0 rows affected (0.14 sec)
mysql> create table child (
-> id int,
-> parent_id INT,
-> index par_ind (parent_id),
-> foreign key (parent_id)
-> references parent(id)
-> on delete cascade on update cascade -- 比官网例子增加 update cascade
-> ) engine=innodb;
Query ok, 0 rows affected (0.15 sec)
mysql> insert into child values(1,1); -- 我们插入一条数据，id=1，parent_id=1
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`burn_test`.`child`, CONSTRAINT `child_ibfk_1` FOREIGN KEY (`parent_id`) REFERENCES `parent` (`id`) ON DELETE CASCADE)
-- 直接报错了，因为此时parent表中没有任何记录
mysql> insert into parent values(1); -- 现在parent中插入记录
Query OK, 1 row affected (0.03 sec)
mysql> insert into child values(1,1); -- 然后在child中插入记录，且parent_id是在parent中存在的
Query OK, 1 row affected (0.02 sec)
mysql> insert into child values(1,2); -- 插入parent_id=2的记录，报错。因为此时parent_id=2的记录不存在
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`burn_test`.`child`, CONSTRAINT `child_ibfk_1` FOREIGN KEY (`parent_id`) REFERENCES `parent` (`id`) ON DELETE CASCADE)
mysql> select * from child;
+------+-----------+
| id | parent_id |
+------+-----------+
| 1 | 1 | -- parent_id = 1
+------+-----------+
1 row in set (0.00 sec)
mysql> select * from parent;
+----+
| id |
+----+
| 1 | -- 根据表结构的定义（Foreign_key），这个值就是 child表中的id
+----+
1 row in set (0.00 sec)
mysql> update parent set id=100 where id=1;
Query OK, 1 row affected (0.04 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from parent;
+-----+
| id |
+-----+
| 100 | -- 已经设置成了100
+-----+
1 row in set (0.00 sec)
mysql> select * from child;
+------+-----------+
| id | parent_id |
+------+-----------+
| 1 | 100 | -- 自动变化，这是on update cascade的作用，联级更新，parent更新，child也跟着更新
+------+-----------+
1 row in set (0.00 sec)
mysql> delete from parent where id=100; -- 删除这条记录
Query OK, 1 row affected (0.03 sec)
mysql> select * from parent; -- id=100的记录已经被删除了
Empty set (0.00 sec)
mysql> select * from child; -- id=1，parent_id=100的记录跟着被删除了。on delete cascade的作用
Empty set (0.00 sec)
mysql> alter table child drop foreign key child_ibfk_1; -- 删除 之前的外键
Query OK, 0 rows affected (0.07 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> alter table child add foreign key(parent_id)
-> references parent(id) on update cascade on delete restrict; -- 使用严格模式
Query OK, 0 rows affected (0.27 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> insert into parent values(50);
Query OK, 1 row affected (0.03 sec)
mysql> insert into child values(3,50);
Query OK, 1 row affected (0.03 sec)
mysql> insert into child values(3,51); -- 和之前一样会提示错误
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`burn_test`.`child`, CONSTRAINT `child_ibfk_1` FOREIGN KEY (`parent_id`) REFERENCES `parent` (`id`) ON UPDATE CASCADE)
mysql> delete from parent where id=50; -- 删除失败了，因为是restrict模式
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`burn_test`.`child`, CONSTRAINT `child_ibfk_1` FOREIGN KEY (`parent_id`) REFERENCES `parent` (`id`) ON UPDATE CASCADE)
注：no action == restrict
外键约束，可以让数据进行一致性更新，但是会有一定的性能损耗，线上业务使用不多。通常上述级联更新和删除都是由应用层业务逻辑进行判断并实现。
表修改
语法
ALTER TABLE tbl_name
[alter_specification [, alter_specification] ...]
[partition_options]
alter_specification:
table_options
| ADD [COLUMN] col_name column_definition
[FIRST | AFTER col_name]
| ADD [COLUMN] (col_name column_definition,...)
| ADD {INDEX|KEY} [index_name]
[index_type] (key_part,...) [index_option] ...
| ADD {FULLTEXT|SPATIAL} [INDEX|KEY] [index_name]
(key_part,...) [index_option] ...
| ADD [CONSTRAINT [symbol]] PRIMARY KEY
[index_type] (key_part,...)
[index_option] ...
| ADD [CONSTRAINT [symbol]] UNIQUE [INDEX|KEY]
[index_name] [index_type] (key_part,...)
[index_option] ...
| ADD [CONSTRAINT [symbol]] FOREIGN KEY
[index_name] (col_name,...)
reference_definition
| ADD CHECK (expr)
| ALGORITHM [=] {DEFAULT|INPLACE|COPY}
| ALTER [COLUMN] col_name {SET DEFAULT literal | DROP DEFAULT}
| CHANGE [COLUMN] old_col_name new_col_name column_definition
[FIRST|AFTER col_name]
| [DEFAULT] CHARACTER SET [=] charset_name [COLLATE [=] collation_name]
| CONVERT TO CHARACTER SET charset_name [COLLATE collation_name]
| {DISABLE|ENABLE} KEYS
| {DISCARD|IMPORT} TABLESPACE
| DROP [COLUMN] col_name
| DROP {INDEX|KEY} index_name
| DROP PRIMARY KEY
| DROP FOREIGN KEY fk_symbol
| FORCE
| LOCK [=] {DEFAULT|NONE|SHARED|EXCLUSIVE}
| MODIFY [COLUMN] col_name column_definition
[FIRST | AFTER col_name]
| ORDER BY col_name [, col_name] ...
| RENAME {INDEX|KEY} old_index_name TO new_index_name
| RENAME [TO|AS] new_tbl_name
| {WITHOUT|WITH} VALIDATION
| ADD PARTITION (partition_definition)
| DROP PARTITION partition_names
| DISCARD PARTITION {partition_names | ALL} TABLESPACE
| IMPORT PARTITION {partition_names | ALL} TABLESPACE
| TRUNCATE PARTITION {partition_names | ALL}
| COALESCE PARTITION number
| REORGANIZE PARTITION partition_names INTO (partition_definitions)
| EXCHANGE PARTITION partition_name WITH TABLE tbl_name [{WITH|WITHOUT} VALIDATION]
| ANALYZE PARTITION {partition_names | ALL}
| CHECK PARTITION {partition_names | ALL}
| OPTIMIZE PARTITION {partition_names | ALL}
| REBUILD PARTITION {partition_names | ALL}
| REPAIR PARTITION {partition_names | ALL}
| REMOVE PARTITIONING
| UPGRADE PARTITIONING
key_part:
col_name [(length)] [ASC | DESC]
index_type:
USING {BTREE | HASH}
index_option:
KEY_BLOCK_SIZE [=] value
| index_type
| WITH PARSER parser_name
| COMMENT 'string'
table_options:
table_option [[,] table_option] ...
table_option:
AUTO_INCREMENT [=] value
| AVG_ROW_LENGTH [=] value
| [DEFAULT] CHARACTER SET [=] charset_name
| CHECKSUM [=] {0 | 1}
| [DEFAULT] COLLATE [=] collation_name
| COMMENT [=] 'string'
| COMPRESSION [=] {'ZLIB'|'LZ4'|'NONE'}
| CONNECTION [=] 'connect_string'
| {DATA|INDEX} DIRECTORY [=] 'absolute path to directory'
| DELAY_KEY_WRITE [=] {0 | 1}
| ENCRYPTION [=] {'Y' | 'N'}
| ENGINE [=] engine_name
| INSERT_METHOD [=] { NO | FIRST | LAST }
| KEY_BLOCK_SIZE [=] value
| MAX_ROWS [=] value
| MIN_ROWS [=] value
| PACK_KEYS [=] {0 | 1 | DEFAULT}
| PASSWORD [=] 'string'
| ROW_FORMAT [=] {DEFAULT|DYNAMIC|FIXED|COMPRESSED|REDUNDANT|COMPACT}
| STATS_AUTO_RECALC [=] {DEFAULT|0|1}
| STATS_PERSISTENT [=] {DEFAULT|0|1}
| STATS_SAMPLE_PAGES [=] value
| TABLESPACE tablespace_name [STORAGE {DISK|MEMORY}]
| UNION [=] (tbl_name[,tbl_name]...)
partition_options:
(see CREATE TABLE options)
操作
mysql> create table a ( a int );
Query OK, 0 rows affected (0.09 sec)
mysql> show create table a\G
*************************** 1. row ***************************
Table: a
Create Table: CREATE TABLE `a` (
`a` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4
1 row in set (0.00 sec)
# 增加一列 b
mysql> alter table a add column b char(11);
Query OK, 0 rows affected (0.20 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> show create table a\G
*************************** 1. row ***************************
Table: a
Create Table: CREATE TABLE `a` (
`a` int(11) DEFAULT NULL,
`b` char(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4
1 row in set (0.00 sec)
mysql> select * from a;
Empty set (0.00 sec)
mysql> insert into a select 1, 'asd';
Query OK, 1 row affected (0.01 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql> select * from a;
+------+------+
| a | b |
+------+------+
| 1 | asd |
+------+------+
1 row in set (0.00 sec)
# 删除 a 列
mysql> alter table a drop column a;
Query OK, 0 rows affected (0.06 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> select * from a;
+------+
| b |
+------+
| asd |
+------+
1 row in set (0.00 sec)
mysql>
注意，当表记录很大的时候，alter table会很耗时，影响性能
安装Employees
Employees官方文档
Employees介绍
Employees数据库是一个用于学习和测试的数据库，大约160MB，4百万条记录
下载
git clone https://github.com/datacharmer/test_db.git
查看
cd test_db
ls -al
[root@iZwz956snfyrvah6yq8sa4Z test_db]# ll
总用量 168344
-rw-r--r-- 1 root root 964 6月 9 13:40 Changelog
-rw-r--r-- 1 root root 7948 6月 9 13:40 employees_partitioned_5.1.sql
-rw-r--r-- 1 root root 6276 6月 9 13:40 employees_partitioned.sql
-rw-r--r-- 1 root root 4193 6月 9 13:40 employees.sql
drwxr-xr-x 2 root root 4096 6月 9 13:40 images
-rw-r--r-- 1 root root 250 6月 9 13:40 load_departments.dump
-rw-r--r-- 1 root root 14159880 6月 9 13:40 load_dept_emp.dump
-rw-r--r-- 1 root root 1090 6月 9 13:40 load_dept_manager.dump
-rw-r--r-- 1 root root 17722832 6月 9 13:40 load_employees.dump
-rw-r--r-- 1 root root 39806034 6月 9 13:40 load_salaries1.dump
-rw-r--r-- 1 root root 39805981 6月 9 13:40 load_salaries2.dump
-rw-r--r-- 1 root root 39080916 6月 9 13:40 load_salaries3.dump
-rw-r--r-- 1 root root 21708736 6月 9 13:40 load_titles.dump
-rw-r--r-- 1 root root 4568 6月 9 13:40 objects.sql
-rw-r--r-- 1 root root 4325 6月 9 13:40 README.md
drwxr-xr-x 2 root root 4096 6月 9 13:40 sakila
-rw-r--r-- 1 root root 272 6月 9 13:40 show_elapsed.sql
-rwxr-xr-x 1 root root 1800 6月 9 13:40 sql_test.sh
-rw-r--r-- 1 root root 4878 6月 9 13:40 test_employees_md5.sql
-rw-r--r-- 1 root root 4882 6月 9 13:40 test_employees_sha.sql
安装
[root@iZwz956snfyrvah6yq8sa4Z test_db]# mysql -uroot -p -t < employees.sql
Enter password:
+-----------------------------+
| INFO |
+-----------------------------+
| CREATING DATABASE STRUCTURE |
+-----------------------------+
+------------------------+
| INFO |
+------------------------+
| storage engine: InnoDB |
+------------------------+
+---------------------+
| INFO |
+---------------------+
| LOADING departments |
+---------------------+
+-------------------+
| INFO |
+-------------------+
| LOADING employees |
+-------------------+
+------------------+
| INFO |
+------------------+
| LOADING dept_emp |
+------------------+
+----------------------+
| INFO |
+----------------------+
| LOADING dept_manager |
+----------------------+
+----------------+
| INFO |
+----------------+
| LOADING titles |
+----------------+
+------------------+
| INFO |
+------------------+
| LOADING salaries |
+------------------+
+---------------------+
| data_load_time_diff |
+---------------------+
| 00:01:14 |
+---------------------+
[root@iZwz956snfyrvah6yq8sa4Z test_db]#
验证
[root@iZwz956snfyrvah6yq8sa4Z test_db]# mysql -uroot -p -t < test_employees_sha.sql
Enter password:
+----------------------+
| INFO |
+----------------------+
| TESTING INSTALLATION |
+----------------------+
+--------------+------------------+------------------------------------------+
| table_name | expected_records | expected_crc |
+--------------+------------------+------------------------------------------+
| employees | 300024 | 4d4aa689914d8fd41db7e45c2168e7dcb9697359 |
| departments | 9 | 4b315afa0e35ca6649df897b958345bcb3d2b764 |
| dept_manager | 24 | 9687a7d6f93ca8847388a42a6d8d93982a841c6c |
| dept_emp | 331603 | d95ab9fe07df0865f592574b3b33b9c741d9fd1b |
| titles | 443308 | d12d5f746b88f07e69b9e36675b6067abb01b60e |
| salaries | 2844047 | b5a1785c27d75e33a4173aaa22ccf41ebd7d4a9f |
+--------------+------------------+------------------------------------------+
+--------------+------------------+------------------------------------------+
| table_name | found_records | found_crc |
+--------------+------------------+------------------------------------------+
| employees | 300024 | 4d4aa689914d8fd41db7e45c2168e7dcb9697359 |
| departments | 9 | 4b315afa0e35ca6649df897b958345bcb3d2b764 |
| dept_manager | 24 | 9687a7d6f93ca8847388a42a6d8d93982a841c6c |
| dept_emp | 331603 | d95ab9fe07df0865f592574b3b33b9c741d9fd1b |
| titles | 443308 | d12d5f746b88f07e69b9e36675b6067abb01b60e |
| salaries | 2844047 | b5a1785c27d75e33a4173aaa22ccf41ebd7d4a9f |
+--------------+------------------+------------------------------------------+
+--------------+---------------+-----------+
| table_name | records_match | crc_match |
+--------------+---------------+-----------+
| employees | OK | ok |
| departments | OK | ok |
| dept_manager | OK | ok |
| dept_emp | OK | ok |
| titles | OK | ok |
| salaries | OK | ok |
+--------------+---------------+-----------+
+------------------+
| computation_time |
+------------------+
| 00:00:16 |
+------------------+
+---------+--------+
| summary | result |
+---------+--------+
| CRC | OK |
| count | OK |
+---------+--------+
[root@iZwz956snfyrvah6yq8sa4Z test_db]# mysql -uroot -p -t < test_employees_md5.sql
Enter password:
+----------------------+
| INFO |
+----------------------+
| TESTING INSTALLATION |
+----------------------+
+--------------+------------------+----------------------------------+
| table_name | expected_records | expected_crc |
+--------------+------------------+----------------------------------+
| employees | 300024 | 4ec56ab5ba37218d187cf6ab09ce1aa1 |
| departments | 9 | d1af5e170d2d1591d776d5638d71fc5f |
| dept_manager | 24 | 8720e2f0853ac9096b689c14664f847e |
| dept_emp | 331603 | ccf6fe516f990bdaa49713fc478701b7 |
| titles | 443308 | bfa016c472df68e70a03facafa1bc0a8 |
| salaries | 2844047 | fd220654e95aea1b169624ffe3fca934 |
+--------------+------------------+----------------------------------+
+--------------+------------------+----------------------------------+
| table_name | found_records | found_crc |
+--------------+------------------+----------------------------------+
| employees | 300024 | 4ec56ab5ba37218d187cf6ab09ce1aa1 |
| departments | 9 | d1af5e170d2d1591d776d5638d71fc5f |
| dept_manager | 24 | 8720e2f0853ac9096b689c14664f847e |
| dept_emp | 331603 | ccf6fe516f990bdaa49713fc478701b7 |
| titles | 443308 | bfa016c472df68e70a03facafa1bc0a8 |
| salaries | 2844047 | fd220654e95aea1b169624ffe3fca934 |
+--------------+------------------+----------------------------------+
+--------------+---------------+-----------+
| table_name | records_match | crc_match |
+--------------+---------------+-----------+
| employees | OK | ok |
| departments | OK | ok |
| dept_manager | OK | ok |
| dept_emp | OK | ok |
| titles | OK | ok |
| salaries | OK | ok |
+--------------+---------------+-----------+
+------------------+
| computation_time |
+------------------+
| 00:00:14 |
+------------------+
+---------+--------+
| summary | result |
+---------+--------+
| CRC | OK |
| count | OK |
+---------+--------+
[root@iZwz956snfyrvah6yq8sa4Z test_db]#
至此，Employees测试数据库安装成功
表查询
语法
SELECT
[ALL | DISTINCT | DISTINCTROW ]
[HIGH_PRIORITY]
[STRAIGHT_JOIN]
[SQL_SMALL_RESULT] [SQL_BIG_RESULT] [SQL_BUFFER_RESULT]
[SQL_CACHE | SQL_NO_CACHE] [SQL_CALC_FOUND_ROWS]
select_expr [, select_expr ...]
[FROM table_references
[PARTITION partition_list]
[WHERE where_condition]
[GROUP BY {col_name | expr | position}
[ASC | DESC], ... [WITH ROLLUP]]
[HAVING where_condition]
[ORDER BY {col_name | expr | position}
[ASC | DESC], ...]
[LIMIT {[offset,] row_count | row_count OFFSET offset}]
[PROCEDURE procedure_name(argument_list)]
[INTO OUTFILE 'file_name'
[CHARACTER SET charset_name]
export_options
| INTO DUMPFILE 'file_name'
| INTO var_name [, var_name]]
[FOR UPDATE | LOCK IN SHARE MODE]]
操作
mysql> select host,user from user;
+-----------+---------------+
| host | user |
+-----------+---------------+
| 127.0.0.1 | chase |
| 127.0.0.1 | jim |
| localhost | mysql.session |
| localhost | mysql.sys |
| localhost | root |
+-----------+---------------+
5 rows in set (0.00 sec)
mysql>
LIMIT 和 ORDER BY
mysql> select * from employees limit 1; -- 从employees中 随机 取出一条数据，结果是不确定的
+--------+------------+------------+-----------+--------+------------+
| emp_no | birth_date | first_name | last_name | gender | hire_date |
+--------+------------+------------+-----------+--------+------------+
| 10001 | 1953-09-02 | Georgi | Facello | M | 1986-06-26 |
+--------+------------+------------+-----------+--------+------------+
1 row in set (0.00 sec)
--
-- order by col_name 根据某列的值进行排序
-- asc ： 升序(default)
-- desc： 降序
--
mysql> select * from employees order by emp_no asc limit 1; -- 使用order by col_name asc进行升序排序
+--------+------------+------------+-----------+--------+------------+
| emp_no | birth_date | first_name | last_name | gender | hire_date |
+--------+------------+------------+-----------+--------+------------+
| 10001 | 1953-09-02 | Georgi | Facello | M | 1986-06-26 |
+--------+------------+------------+-----------+--------+------------+
1 row in set (0.00 sec)
mysql> select * from employees order by emp_no limit 1; -- 默认就是升序的
+--------+------------+------------+-----------+--------+------------+
| emp_no | birth_date | first_name | last_name | gender | hire_date |
+--------+------------+------------+-----------+--------+------------+
| 10001 | 1953-09-02 | Georgi | Facello | M | 1986-06-26 | -- 结果和上面一致
+--------+------------+------------+-----------+--------+------------+
1 row in set (0.00 sec)
mysql> select * from employees order by emp_no desc limit 1; -- desc表示降序
+--------+------------+------------+-----------+--------+------------+
| emp_no | birth_date | first_name | last_name | gender | hire_date |
+--------+------------+------------+-----------+--------+------------+
| 499999 | 1958-05-01 | Sachin | Tsukuda | M | 1997-11-30 | -- 降序显示
+--------+------------+------------+-----------+--------+------------+
-- 通过order by排序后 limit 1 才是确定的
1 row in set (0.00 sec)
mysql> show create table employees\G
*************************** 1. row ***************************
Table: employees
Create Table: CREATE TABLE `employees` (
`emp_no` int(11) NOT NULL,
`birth_date` date NOT NULL,
`first_name` varchar(14) NOT NULL,
`last_name` varchar(16) NOT NULL,
`gender` enum('M','F') NOT NULL,
`hire_date` date NOT NULL,
PRIMARY KEY (`emp_no`) -- emp_no 是主键，order by 主键 不会创建临时表的，主键(索引)本身有序
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4
1 row in set (0.00 sec)
mysql> select * from employees order by emp_no asc limit 5,5; -- limit start, offset
-- 从第5条 开始取，取5条出来
+--------+------------+------------+-----------+--------+------------+
| emp_no | birth_date | first_name | last_name | gender | hire_date |
+--------+------------+------------+-----------+--------+------------+
| 10006 | 1953-04-20 | Anneke | Preusig | F | 1989-06-02 |
| 10007 | 1957-05-23 | Tzvetan | Zielinski | F | 1989-02-10 |
| 10008 | 1958-02-19 | Saniya | Kalloufi | M | 1994-09-15 |
| 10009 | 1952-04-19 | Sumant | Peac | F | 1985-02-18 |
| 10010 | 1963-06-01 | Duangkaew | Piveteau | F | 1989-08-24 |
+--------+------------+------------+-----------+--------+------------+
5 rows in set (0.00 sec)
-- 以上这个语法有一种分页的效果，但是会随着start的增加，性能会下降，因为会扫描表(从 1 到 start)
-- 相对比较推荐的方法
mysql> select * from employees where emp_no > 20000 order by emp_no limit 5;
+--------+------------+------------+-----------+--------+------------+
| emp_no | birth_date | first_name | last_name | gender | hire_date |
+--------+------------+------------+-----------+--------+------------+
| 20001 | 1962-05-16 | Atreye | Eppinger | M | 1990-04-18 |
| 20002 | 1955-12-25 | Jaber | Brender | M | 1988-01-26 |
| 20003 | 1953-04-11 | Munehiko | Coors | F | 1991-02-07 |
| 20004 | 1952-03-07 | Radoslaw | Pfau | M | 1995-11-24 |
| 20005 | 1956-02-20 | Licheng | Przulj | M | 1992-07-17 |
+--------+------------+------------+-----------+--------+------------+
5 rows in set (0.00 sec)
-- （当然推荐把热数据放cache里,比如Redis）
ORDER BY 是把已经查询好的结果集进行排序
WHERE
WHERE是将查询出来的结果，通过WHERE后面的条件(condition)，对结果进行过滤
mysql> select * from employees where emp_no > 30000 emp_no limit 4; -- 不加order by的limit是不确定的SQL
+--------+------------+------------+-----------+--------+------------+
| emp_no | birth_date | first_name | last_name | gender | hire_date |
+--------+------------+------------+-----------+--------+------------+
| 30001 | 1953-03-27 | Izaskun | Morton | M | 1988-05-21 |
| 30002 | 1960-08-23 | Branimir | Snedden | M | 1998-09-24 |
| 30003 | 1952-11-25 | Takahito | Vilarrasa | M | 1990-08-22 |
| 30004 | 1957-11-26 | Lucian | Penttonen | F | 1992-10-08 |
+--------+------------+------------+-----------+--------+------------+
4 rows in set (0.00 sec)
mysql> select * from employees where emp_no > 40000 order by emp_no limit 4;
+--------+------------+------------+-----------+--------+------------+
| emp_no | birth_date | first_name | last_name | gender | hire_date |
+--------+------------+------------+-----------+--------+------------+
| 40001 | 1956-03-28 | Akemi | Maliniak | F | 1987-08-06 |
| 40002 | 1960-03-15 | Nakhoon | Badr | M | 1990-02-13 |
| 40003 | 1960-01-26 | Jacopo | Marshall | F | 1991-09-30 |
| 40004 | 1955-09-09 | Anneke | Stiles | F | 1986-03-05 |
+--------+------------+------------+-----------+--------+------------+
4 rows in set (0.02 sec)
mysql> select * from employees
-> where emp_no > 40000
-> and hire_date > '1991-01-01' -- 可以用 and 进行 逻辑与
-> order by emp_no limit 4;
+--------+------------+------------+------------+--------+------------+
| emp_no | birth_date | first_name | last_name | gender | hire_date |
+--------+------------+------------+------------+--------+------------+
| 40003 | 1960-01-26 | Jacopo | Marshall | F | 1991-09-30 |
| 40005 | 1961-02-27 | Zsolt | Fairtlough | F | 1991-07-08 |
| 40012 | 1955-02-07 | Chinhyun | Ozeri | F | 1995-08-12 |
| 40015 | 1964-10-08 | Ioana | Lemarechal | M | 1997-08-07 |
+--------+------------+------------+------------+--------+------------+
4 rows in set (0.00 sec)
mysql> select * from employees
-> where (emp_no > 40000 and birth_date > '1961-01-01') -- 使用()明确条件的逻辑规则
-> or (emp_no > 40000 and hire_date > '1991-01-01') -- 可以使用 or 做 逻辑或
-> order by emp_no limit 5;
+--------+------------+------------+------------+--------+------------+
| emp_no | birth_date | first_name | last_name | gender | hire_date |
+--------+------------+------------+------------+--------+------------+
| 40003 | 1960-01-26 | Jacopo | Marshall | F | 1991-09-30 |
| 40005 | 1961-02-27 | Zsolt | Fairtlough | F | 1991-07-08 |
| 40006 | 1962-11-07 | Basim | Panienski | F | 1986-12-27 |
| 40012 | 1955-02-07 | Chinhyun | Ozeri | F | 1995-08-12 |
| 40015 | 1964-10-08 | Ioana | Lemarechal | M | 1997-08-07 |
+--------+------------+------------+------------+--------+------------+
5 rows in set (0.00 sec)
JOIN
image.png
--
-- ANSI SQL 89
-- 关联employees表和titles表
-- 要求是 employees的emp_no 等于 titles的emp_no
--
mysql> select * from employees,titles where employees.emp_no = titles.emp_no limit 5;
+--------+------------+------------+-----------+--------+------------+--------+-----------------+------------+------------+
| emp_no | birth_date | first_name | last_name | gender | hire_date | emp_no | title | from_date | to_date |
+--------+------------+------------+-----------+--------+------------+--------+-----------------+------------+------------+
| 10001 | 1953-09-02 | Georgi | Facello | M | 1986-06-26 | 10001 | Senior Engineer | 1986-06-26 | 9999-01-01 |
| 10002 | 1964-06-02 | Bezalel | Simmel | F | 1985-11-21 | 10002 | Staff | 1996-08-03 | 9999-01-01 |
| 10003 | 1959-12-03 | Parto | Bamford | M | 1986-08-28 | 10003 | Senior Engineer | 1995-12-03 | 9999-01-01 |
| 10004 | 1954-05-01 | Chirstian | Koblick | M | 1986-12-01 | 10004 | Engineer | 1986-12-01 | 1995-12-01 |
| 10004 | 1954-05-01 | Chirstian | Koblick | M | 1986-12-01 | 10004 | Senior Engineer | 1995-12-01 | 9999-01-01 |
+--------+------------+------------+-----------+--------+------------+--------+-----------------+------------+------------+
5 rows in set (0.00 sec)
--
-- 在上面的基础上只显示emp_no，名字，性别和职位名称
--
mysql> select emp_no, concat(last_name,' ', first_name), gender, title
-> from employees,titles
-> where employees.emp_no = titles.emp_no limit 5;
ERROR 1052 (23000): Column 'emp_no' in field list is ambiguous -- 报错了，原因是emp_no两个表都有
mysql> select employees.emp_no, -- 指定了employees
-> concat(last_name,' ', first_name), gender, title
-> from employees,titles
-> where employees.emp_no = titles.emp_no limit 5;
+--------+-----------------------------------+--------+-----------------+
| emp_no | concat(last_name,' ', first_name) | gender | title |
+--------+-----------------------------------+--------+-----------------+
| 10001 | Facello Georgi | M | Senior Engineer |
| 10002 | Simmel Bezalel | F | Staff |
| 10003 | Bamford Parto | M | Senior Engineer |
| 10004 | Koblick Chirstian | M | Engineer |
| 10004 | Koblick Chirstian | M | Senior Engineer |
+--------+-----------------------------------+--------+-----------------+
mysql> select employees.emp_no,
-> concat(last_name,' ', first_name) as emp_name, gender, title -- 对名字的列取一个别名叫emp_name
-> from employees,titles
-> where employees.emp_no = titles.emp_no limit 5;
+--------+-------------------+--------+-----------------+
| emp_no | emp_name | gender | title | -- 这里就显示了emp_name
+--------+-------------------+--------+-----------------+
| 10001 | Facello Georgi | M | Senior Engineer |
| 10002 | Simmel Bezalel | F | Staff |
| 10003 | Bamford Parto | M | Senior Engineer |
| 10004 | Koblick Chirstian | M | Engineer |
| 10004 | Koblick Chirstian | M | Senior Engineer |
+--------+-------------------+--------+-----------------+
5 rows in set (0.00 sec)
mysql> select e.emp_no, -- 使用表的别名
-> concat(last_name,' ', first_name) as emp_name, gender, title
-> from employees as e,titles as t -- 对表做别名
-> where e.emp_no = t.emp_no limit 5; -- 使用报表的别名
+--------+-------------------+--------+-----------------+
| emp_no | emp_name | gender | title |
+--------+-------------------+--------+-----------------+
| 10001 | Facello Georgi | M | Senior Engineer |
| 10002 | Simmel Bezalel | F | Staff |
| 10003 | Bamford Parto | M | Senior Engineer |
| 10004 | Koblick Chirstian | M | Engineer |
| 10004 | Koblick Chirstian | M | Senior Engineer |
+--------+-------------------+--------+-----------------+
5 rows in set (0.00 sec)
--
-- ANSI SQL 92
-- inner join ... on ...语法
--
mysql> select e.emp_no,
-> concat(last_name,' ', first_name) as emp_name, gender, title
-> from employees as e inner join titles as t -- inner join 可以省略inner关键字
-> on e.emp_no = t.emp_no limit 5; -- 配合join使用on
+--------+-------------------+--------+-----------------+
| emp_no | emp_name | gender | title |
+--------+-------------------+--------+-----------------+
| 10001 | Facello Georgi | M | Senior Engineer |
| 10002 | Simmel Bezalel | F | Staff |
| 10003 | Bamford Parto | M | Senior Engineer |
| 10004 | Koblick Chirstian | M | Engineer |
| 10004 | Koblick Chirstian | M | Senior Engineer |
+--------+-------------------+--------+-----------------+
5 rows in set (0.00 sec)
--
-- 上面两种语句在效率上其实是一样的，只是语法上的区别
--
--- 第一种
mysql> explain select e.emp_no,
-> concat(last_name,' ', first_name) as emp_name, gender, title
-> from employees as e,titles as t
-> where e.emp_no = t.emp_no limit 5;
+----+-------------+-------+------------+------+---------------+---------+---------+--------------------+--------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+---------+---------+--------------------+--------+----------+-------------+
| 1 | SIMPLE | e | NULL | ALL | PRIMARY | NULL | NULL | NULL | 298124 | 100.00 | NULL |
| 1 | SIMPLE | t | NULL | ref | PRIMARY | PRIMARY | 4 | employees.e.emp_no | 1 | 100.00 | Using index |
+----+-------------+-------+------------+------+---------------+---------+---------+--------------------+--------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
--- 第二种
mysql> explain select e.emp_no,
-> concat(last_name,' ', first_name) as emp_name, gender, title
-> from employees as e inner join titles as t
-> on e.emp_no = t.emp_no limit 5;
+----+-------------+-------+------------+------+---------------+---------+---------+--------------------+--------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+---------+---------+--------------------+--------+----------+-------------+
| 1 | SIMPLE | e | NULL | ALL | PRIMARY | NULL | NULL | NULL | 298124 | 100.00 | NULL |
| 1 | SIMPLE | t | NULL | ref | PRIMARY | PRIMARY | 4 | employees.e.emp_no | 1 | 100.00 | Using index |
+----+-------------+-------+------------+------+---------------+---------+---------+--------------------+--------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
-- 通过explain看两条语句的执行计划，发现是一样的，所以性能上是一样的，只是语法的不同
OUTER JOIN
--
-- 左连接 left join
--
mysql> use burn_test
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
Database changed
mysql> create table test_left_join_1(a int);
Query OK, 0 rows affected (0.16 sec)
mysql> create table test_left_join_2(b int);
Query OK, 0 rows affected (0.14 sec)
mysql> insert into test_left_join_1 values (1);
Query OK, 1 row affected (0.03 sec)
mysql> insert into test_left_join_1 values (2);
Query OK, 1 row affected (0.03 sec)
mysql> insert into test_left_join_2 values (1);
Query OK, 1 row affected (0.03 sec)
mysql> select * from test_left_join_1;
+------+
| a |
+------+
| 1 |
| 2 |
+------+
2 rows in set (0.00 sec)
mysql> select * from test_left_join_2;
+------+
| b |
+------+
| 1 |
+------+
1 row in set (0.00 sec)
mysql> select * from
-> test_left_join_1 as t1
-> left join -- 使用left join
-> test_left_join_2 as t2
-> on t1.a = t2.b;
+------+------+
| a | b |
+------+------+
| 1 | 1 | -- 满足条件的，显示t2中该条记录的值
| 2 | NULL | -- 不满足条件的，用NULL填充
+------+------+
2 rows in set (0.00 sec)
-- left join ： 左表 left join 右表 on 条件；
-- 左表全部显示，右表是匹配表，
-- 如果右表的某条记录满足 [on 条件] 匹配，则该记录显示
-- 如果右表的某条记录 不 满足 匹配，则该记录显示NULL
--
-- 右连接 right join （继续使用test_left_join_1和2两张表）
--
mysql> select * from
-> test_left_join_1 as t1
-> right join -- 使用right join
-> test_left_join_2 as t2
-> on t1.a = t2.b;
+------+------+
| a | b |
+------+------+
| 1 | 1 | -- 右表（t2）全部显示
+------+------+
1 row in set (0.00 sec)
-- right join ： 左表 right join 右表 on 条件
-- 右表全部显示，左边是匹配表
-- 同样和left join，满足则显示，不满足且右表中有值，则填充NULL
mysql> insert into test_left_join_2 values (3); -- t2 中再增加一条记录
Query OK, 1 row affected (0.03 sec)
mysql> select * from
-> test_left_join_1 as t1
-> right join
-> test_left_join_2 as t2
-> on t1.a = t2.b;
+------+------+
| a | b |
+------+------+
| 1 | 1 |
| NULL | 3 | -- 右表存在，左表没有，用NULL填充
+------+------+
2 rows in set (0.00 sec)
--
-- 查找在t1表，而不在t2表的数据
--
mysql> select * from
-> test_left_join_1 as t1
-> left join
-> test_left_join_2 as t2
-> on t1.a = t2.b where t2.b is null;
+------+------+
| a | b |
+------+------+
| 2 | NULL | -- 数据1 在t1和t2中都有，所以不显示
+------+------+
1 row in set (0.00 sec)
-- left join ： left outer join , outer关键字可以省略
-- right join： right outer join , outer 关键字可以省略
-- join无论inner还是outer，列名不需要一样，甚至列的类型也可以不一样，会进行转换。
-- 一般情况下，表设计合理，需要关联的字段类型应该是一样的
--
-- 查找哪些员工不是经理
--
mysql> select e.emp_no,
-> concat(last_name,' ', first_name) as emp_name, gender, d.dept_no
-> from employees as e left join dept_manager as d
-> on e.emp_no = d.emp_no
-> where d.emp_no is null limit 5;
+--------+-------------------+--------+---------+
| emp_no | emp_name | gender | dept_no | -- dept_no是dept_manager的字段
+--------+-------------------+--------+---------+
| 10001 | Facello Georgi | M | NULL |
| 10002 | Simmel Bezalel | F | NULL |
| 10003 | Bamford Parto | M | NULL |
| 10004 | Koblick Chirstian | M | NULL |
| 10005 | Maliniak Kyoichi | M | NULL |
+--------+-------------------+--------+---------+
5 rows in set (0.00 sec)
-- 在 inner join中，过滤条件放在where或者on中都是可以的
-- 在 outer join中 条件放在where和on中是不一样的
mysql> select * from
-> test_left_join_1 as t1
-> left join
-> test_left_join_2 as t2
-> on t1.a = t2.b
-> where t2.b is null;
+------+------+
| a | b |
+------+------+
| 2 | NULL |
+------+------+
1 row in set (0.00 sec)
mysql> select * from
-> test_left_join_1 as t1
-> left join
-> test_left_join_2 as t2
-> on t1.a = t2.b
-> and t2.b is null; -- 除了a=b, 还要找到b=null的，但是b里面没有null，所有a全部显示，b全为null
+------+------+
| a | b |
+------+------+
| 1 | NULL |
| 2 | NULL |
+------+------+
2 rows in set (0.00 sec)
-- ON 参与outer join的结果的生成，而where只是对结果的一个过滤
GROUP BY
--
-- 找出同一个部门的员工数量
--
mysql> select dept_no, count(dept_no) -- count是得到数量，这里就是分组函数
-> from dept_emp
-> group by dept_no; -- 通过 dept_no 分组
+---------+----------------+
| dept_no | count(dept_no) |
+---------+----------------+
| d001 | 20211 |
| d002 | 17346 |
| d003 | 17786 |
| d004 | 73485 |
| d005 | 85707 |
| d006 | 20117 |
| d007 | 52245 |
| d008 | 21126 |
| d009 | 23580 |
+---------+----------------+
9 rows in set (0.10 sec)
--
-- 选出部门人数 > 50000
--
mysql> select dept_no, count(dept_no)
-> from dept_emp
-> group by dept_no
-> having count(dept_no) > 50000; -- 如果是对分组的聚合函数做过滤，使用having，用where报语法错误
+---------+----------------+
| dept_no | count(dept_no) |
+---------+----------------+
| d004 | 73485 |
| d005 | 85707 |
| d007 | 52245 |
+---------+----------------+
3 rows in set (0.09 sec)
--
-- 每个用户每个月产生的订单数目
--
mysql> desc orders;
+-----------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------------+-------------+------+-----+---------+-------+
| o_orderkey | int(11) | NO | PRI | NULL | | -- 订单ID
| o_custkey | int(11) | YES | MUL | NULL | | -- 客户ID
| o_orderstatus | char(1) | YES | | NULL | |
| o_totalprice | double | YES | | NULL | |
| o_orderDATE | date | YES | MUL | NULL | | -- 订单日期
| o_orderpriority | char(15) | YES | | NULL | |
| o_clerk | char(15) | YES | | NULL | |
| o_shippriority | int(11) | YES | | NULL | |
| o_comment | varchar(79) | YES | | NULL | |
+-----------------+-------------+------+-----+---------+-------+
9 rows in set (0.00 sec)
mysql> select o_orderkey, o_custkey, o_orderDATE from orders limit 3;
+------------+-----------+-------------+
| o_orderkey | o_custkey | o_orderDATE |
+------------+-----------+-------------+
| 1 | 36901 | 1996-01-02 |
| 2 | 78002 | 1996-12-01 |
| 3 | 123314 | 1993-10-14 |
+------------+-----------+-------------+
3 rows in set (0.00 sec)
--
-- 查找客户每年每月产生的订单数
--
mysql> select o_custkey, count(o_orderkey), -> year(o_orderDATE), month(o_orderDATE)
-> from orders
-> group by o_custkey, year(o_orderDATE), month(o_orderDATE)
-> limit 10;
+-----------+-------------------+-------------------+--------------------+
| o_custkey | count(o_orderkey) | year(o_orderDATE) | month(o_orderDATE) |
+-----------+-------------------+-------------------+--------------------+
| 1 | 1 | 1992 | 4 |
| 1 | 1 | 1992 | 8 |
| 1 | 1 | 1996 | 6 |
| 1 | 1 | 1996 | 7 |
| 1 | 1 | 1996 | 12 |
| 1 | 1 | 1997 | 3 |
| 2 | 1 | 1992 | 4 |
| 2 | 1 | 1994 | 5 |
| 2 | 1 | 1994 | 8 |
| 2 | 1 | 1994 | 12 |
+-----------+-------------------+-------------------+--------------------+
10 rows in set (8.97 sec)
-- 使用 date_format 函数
mysql> select o_custkey, count(o_orderkey),
-> date_format(o_orderDATE, '%Y-%m') as date
-> from orders
-> group by o_custkey, date_format(o_orderDATE, '%Y-%m')
-> limit 10;
+-----------+-------------------+---------+
| o_custkey | count(o_orderkey) | date |
+-----------+-------------------+---------+
| 1 | 1 | 1992-04 |
| 1 | 1 | 1992-08 |
| 1 | 1 | 1996-06 |
| 1 | 1 | 1996-07 |
| 1 | 1 | 1996-12 |
| 1 | 1 | 1997-03 |
| 2 | 1 | 1992-04 |
| 2 | 1 | 1994-05 |
| 2 | 1 | 1994-08 |
| 2 | 1 | 1994-12 |
+-----------+-------------------+---------+
10 rows in set (11.46 sec)
子查询
子查询就是指在一个select语句中嵌套另一个select语句。同时，子查询必须包含括号。
MySQL 5.6.x 版本之前，MySQL的子查询性能较差，但是从5.6开始，不存在性能差的问题。
-
ANY / SOME
如果外部查询的列的结果和子查询的列的结果比较得到为True的话，则返回比较值为True的（外查询）的记录
mysql> create table t1 (a int);
Query OK, 0 rows affected (0.15 sec)
mysql> create table t2 (a int);
Query OK, 0 rows affected (0.14 sec)
mysql> insert into t1 values (4), (10);
Query OK, 2 rows affected (0.00 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> insert into t2 values(5), (12), (15);
Query OK, 3 rows affected (0.01 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> select * from t1;
+------+
| a |
+------+
| 4 |
| 10 |
+------+
2 rows in set (0.00 sec)
mysql> select * from t2;
+------+
| a |
+------+
| 5 |
| 12 |
| 15 |
+------+
3 rows in set (0.00 sec)
mysql> select * from t1 where a > any ( select * from t2 );
+------+
| a |
+------+ -- 10 比 5 大为True，则返回该值，4比t2中所有的a值小，为False
| 10 |
+------+
1 row in set (0.00 sec)
-- 返回(5， 12， 15)
-- t1中a列的值，只要大于(5， 12， 15)中任意一值
-- 即t1.a > t2.a为True，则返回对应的t1.a
-- 这个查询可以解释为，t1表内a列的值 大于 t2表中a列的任意(any)一个值（t1.a > any(t2.a) == true）,则返回t1.a的记录
ANY关键词必须与一个比较操作符一起使用： =, >, <, >=, <=, <> (这个是!=的意思)
子查询中SOME和ANY是同一个意思
-
IN
in是ANY的一种特殊情况："in" equals "= any"
mysql> insert into t2 select 4;
Query OK, 1 row affected (0.00 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql> select * from t1 where a = any( select * from t2 );
+------+
| a |
+------+
| 4 |
+------+
1 row in set (0.00 sec)
mysql>
# in的结果等同于 =any 的结果
mysql> select a from t1 where a in ( select a from t2 );
+------+
| a |
+------+
| 4 |
+------+
1 row in set (0.00 sec)
# select a from s1 where a in (select a in t2);是用的比较多的一种语法
ALL
如果外部查询的列的结果和子查询的列的所有结果比较得到为True的话，则返回比较值为True的（外查询）的记录
# 清空 t1 , t2
mysql> truncate t1;
Query OK, 0 rows affected (0.02 sec)
mysql> truncate table t2;
Query OK, 0 rows affected (0.02 sec)
mysql> insert into t1 values (4), (10);
Query OK, 2 rows affected (0.01 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> insert into t2 values (3), (4), (5);
Query OK, 3 rows affected (0.00 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> select a from t1 where a > all( select a from t2 );
+------+
| a |
+------+
| 10 |
+------+
1 row in set (0.00 sec)
# (10 > 5, 4, 3 为 True) 而 (4 >5, 4, 3 为 False)
mysql>
ALL关键词必须与一个比较操作符一起使用
NOT IN 是 <> ALL的别名
子查询的分类
- 独立子查询
- 不依赖外部查询而运行的子查询
mysql> select * from t1 where a in( 1, 2, 3, 4, 5 );
+------+
| a |
+------+
| 4 |
+------+
1 row in set (0.00 sec)
mysql>
- 相关子查询
- 引用了外部查询列的子查询
mysql> select a from t1 where a in (select * from t2 where t1.a = t2.a);
+------+
| a |
+------+
| 4 |
+------+
1 row in set (0.00 sec)
子查询的优化
- 在MySQL5.6之前，优化器会把子查询重写成exists的形式
select a from t1 where a in (select a from t2);这个是一条独立的子查询，时间复杂度 O(M+N)
经过优化器重写后
select a from t1 where exists (select 1 from t2 where t1.a = t2.a);这是相关子查询，复杂度O(M*N + M)
所以在MySQL 5.6之前，部分的子查询需要重写成join的形式 （注意表的大小）
mysql> select t1.a from t1 join t2 on t1.a = t2.a;
+------+
| a |
+------+
| 4 |
+------+
1 row in set (0.00 sec)
mysql>
- 在MySQL 5.6之后，优化器不会将子查询重写成exists的形式，而是自动优化，性能有了大幅提升
包含NULL值的NOT IN
# MySQL数据库的比较操作，除了返回1(True), 0(False)之外，还会返回NULL
# NULL和NULL的比较，返回的还是NULL
mysql> select null in ( 'a', 'b', null );
+----------------------------+
| null in ( 'a', 'b', null ) |
+----------------------------+
| NULL |
+----------------------------+
1 row in set (0.00 sec)
# null不在('a', 'b', null)中，返回的还是null，因为有null和null的比较
mysql> select null not in ( 'a', 'b', null );
+--------------------------------+
| null not in ( 'a', 'b', null ) |
+--------------------------------+
| NULL |
+--------------------------------+
1 row in set (0.00 sec)
# a 不在 ('a', 'b', null)中，返回0,即False
mysql> select 'a' not in ( 'a', 'b', null );
+-------------------------------+
| 'a' not in ( 'a', 'b', null ) |
+-------------------------------+
| 0 |
+-------------------------------+
1 row in set (0.00 sec)
# 理论上应该是返回1，即True的。但是包含了null值。则返回null
mysql> select 'c' not in ( 'a', 'b', null );
+-------------------------------+
| 'c' not in ( 'a', 'b', null ) |
+-------------------------------+
| NULL |
+-------------------------------+
1 row in set (0.00 sec)
# 这个返回值可以理解 'c'不在('a', 'b')中，返回1，即为True
mysql> select 'c' not in ( 'a', 'b' );
+-------------------------+
| 'c' not in ( 'a', 'b' ) |
+-------------------------+
| 1 |
+-------------------------+
1 row in set (0.00 sec)
mysql>
对于包含了NULL值的IN操作，总是返回True或者NULL
NOT IN返回NOT True (False)或者NOT NULL (NULL)
1.--
2.-- SQL语句一 使用 EXISTS
3.--
4.select customerid, companyname
5. from customers as A
6. where country = 'Spain'
7. and not exists
8. ( select * from orders as B
9. where A.customerid = B.customerid );
10.
11.--
12.-- SQL语句二 使用 IN
13.--
14.select customerid, companyname
15. from customers as A
16. where country = 'Spain'
17. and customerid not in (select customerid from orders);
18.
19.-----
20.-- 当结果集合中没有NULL值时，上述两条SQL语句查询的结果是一致的
21.-----
22.
23.--
24.-- 插入一个NULL值
25.--
26.insert into orders(orderid) values (null);
27.
28.-----
29.-- SQL语句1 : 返回和之前一致
30.-- SQL语句2 : 返回为空表，因为子查询返回的结果集中存在NULL值。not in null 永远返回False或者NULL
31.-- 此时 where (country = 'Spain' and (False or NULL)) 为 False OR NULL，条件永远不匹配
32.-----
33.
34.--
35.-- SQL语句2 改写后
36.--
37.select customerid, companyname
38. from customers as A
39. where country = 'Spain'
40. and customerid not in (select customerid from orders
41. where customerid is not null); -- 增加这个过滤条件，使用is not，而不是<>
42.
43.
44.--
45.-- 和 null比较，使用is和is not， 而不是 = 和 <>
46.--
47.mysql> select null = null;
48.+-------------+
49.| null = null |
50.+-------------+
51.| NULL |
52.+-------------+
53.1 row in set (0.00 sec)
54.
55.mysql> select null <> null;
56.+--------------+
57.| null <> null |
58.+--------------+
59.| NULL |
60.+--------------+
61.1 row in set (0.00 sec)
62.
63.mysql> select null is null;
64.+--------------+
65.| null is null |
66.+--------------+
67.| 1 | -- 返回 True
68.+--------------+
69.1 row in set (0.00 sec)
70.
71.mysql> select null is not null;
72.+-------------------+
73.| null is not null |
74.+-------------------+
75.| 0 | -- 返回 False
76.+-------------------+
77.1 row in set (0.00 sec)
EXISTS不管返回值是什么，而是看是否有行返回，所以EXISTS中子查询都是select *、select 1等，因为只关心返回是否有行（结果集）
INSERT
INSERT [LOW_PRIORITY | DELAYED | HIGH_PRIORITY] [IGNORE]
[INTO] tbl_name
[PARTITION (partition_name [, partition_name] ...)]
[(col_name [, col_name] ...)]
{VALUES | VALUE} (value_list) [, (value_list)] ...
[ON DUPLICATE KEY UPDATE assignment_list]
INSERT [LOW_PRIORITY | DELAYED | HIGH_PRIORITY] [IGNORE]
[INTO] tbl_name
[PARTITION (partition_name [, partition_name] ...)]
SET assignment_list
[ON DUPLICATE KEY UPDATE assignment_list]
INSERT [LOW_PRIORITY | HIGH_PRIORITY] [IGNORE]
[INTO] tbl_name
[PARTITION (partition_name [, partition_name] ...)]
[(col_name [, col_name] ...)]
SELECT ...
[ON DUPLICATE KEY UPDATE assignment_list]
value:
{expr | DEFAULT}
value_list:
value [, value] ...
assignment:
col_name = value
assignment_list:
assignment [, assignment] ...
操作
# 插入一个值
mysql> insert into t1 values(3);
Query OK, 1 row affected (0.01 sec)
# 插入多个值
mysql> insert into t1 values(5),(6),(8);
Query OK, 3 rows affected (0.03 sec)
Records: 3 Duplicates: 0 Warnings: 0
# insert XXX select XXX 语法
mysql> insert into t1 select 23;
Query OK, 1 row affected (0.00 sec)
Records: 1 Duplicates: 0 Warnings: 0
# 有多个列
mysql> create table t3 ( a int, b int );
Query OK, 0 rows affected (0.16 sec)
# 没有指定列，报错
mysql> insert into t3 select 8;
ERROR 1136 (21S01): Column count doesn't match value count at row 1
# 指定 a 列
mysql> insert into t3(a) select 8;
Query OK, 1 row affected (0.01 sec)
Records: 1 Duplicates: 0 Warnings: 0
# 不指定列，但是插入值匹配列的个数和类型
mysql> insert into t3 select 8, 9;
Query OK, 1 row affected (0.01 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql> select * from t3;
+------+------+
| a | b |
+------+------+
| 8 | NULL |
| 8 | 9 |
+------+------+
2 rows in set (0.00 sec)
# 从t2表中查询数据并插入到t3(a)中，注意指定列
mysql> insert into t3(b) select a from t2;
Query OK, 3 rows affected (0.00 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> select * from t3;
+------+------+
| a | b |
+------+------+
| 8 | NULL |
| 8 | 9 |
| NULL | 3 |
| NULL | 4 |
| NULL | 5 |
+------+------+
5 rows in set (0.00 sec)
# 如果想快速增长表格中的数据，可以使用如下方法，使得数据成倍增长
mysql> insert into t3(a) select * from t2;
Query OK, 3 rows affected (0.00 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> select * from t3;
+------+------+
| a | b |
+------+------+
| 8 | NULL |
| 8 | 9 |
| NULL | 3 |
| NULL | 4 |
| NULL | 5 |
| 3 | NULL |
| 4 | NULL |
| 5 | NULL |
+------+------+
8 rows in set (0.00 sec)
mysql> insert into t3(a) select * from t2;
Query OK, 3 rows affected (0.00 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> select * from t3;
+------+------+
| a | b |
+------+------+
| 8 | NULL |
| 8 | 9 |
| NULL | 3 |
| NULL | 4 |
| NULL | 5 |
| 3 | NULL |
| 4 | NULL |
| 5 | NULL |
| 3 | NULL |
| 4 | NULL |
| 5 | NULL |
+------+------+
11 rows in set (0.00 sec)
mysql>
DELETE
DELETE [LOW_PRIORITY] [QUICK] [IGNORE] FROM tbl_name
[PARTITION (partition_name [, partition_name] ...)]
[WHERE where_condition]
[ORDER BY ...]
[LIMIT row_count]
操作
# 根据过滤条件删除
mysql> select * from t3;
+------+------+
| a | b |
+------+------+
| 8 | NULL |
| 8 | 9 |
| NULL | 3 |
| NULL | 4 |
| NULL | 5 |
| 3 | NULL |
| 4 | NULL |
| 5 | NULL |
| 3 | NULL |
| 4 | NULL |
| 5 | NULL |
+------+------+
11 rows in set (0.00 sec)
# 删除整个t3表数据
mysql> delete from t3 where a is null;
Query OK, 3 rows affected (0.01 sec)
mysql> select * from t3;
+------+------+
| a | b |
+------+------+
| 8 | NULL |
| 8 | 9 |
| 3 | NULL |
| 4 | NULL |
| 5 | NULL |
| 3 | NULL |
| 4 | NULL |
| 5 | NULL |
+------+------+
8 rows in set (0.00 sec)
mysql> delete from t3;
Query OK, 8 rows affected (0.01 sec)
mysql> select * from t3;
Empty set (0.00 sec)
mysql>
UPDATE
UPDATE [LOW_PRIORITY] [IGNORE] table_reference
SET assignment_list
[WHERE where_condition]
[ORDER BY ...]
[LIMIT row_count]
value:
{expr | DEFAULT}
assignment:
col_name = value
assignment_list:
assignment [, assignment] ...
操作
mysql> insert into t3 select 1,2;
Query OK, 1 row affected (0.00 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql> select * from t3;
+------+------+
| a | b |
+------+------+
| 1 | 2 |
+------+------+
1 row in set (0.00 sec)
mysql> update t3 set a = 100 where a = 1;
Query OK, 1 row affected (0.02 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from t3;
+------+------+
| a | b |
+------+------+
| 100 | 2 |
+------+------+
1 row in set (0.00 sec)
mysql> select * from t1;
+------+
| a |
+------+
| 4 |
| 10 |
| 1 |
| 3 |
| 5 |
| 6 |
| 8 |
| 23 |
+------+
8 rows in set (0.00 sec)
mysql> select * from t2;
+------+
| a |
+------+
| 3 |
| 4 |
| 5 |
+------+
3 rows in set (0.00 sec)
# 关联更新，先得到t1.a=t2.a的结果集，然后将结果集中的t1.a设置为100
mysql> update t1 join t2 on t1.a = t2.a set t1.a = 100;
Query OK, 3 rows affected (0.03 sec)
Rows matched: 3 Changed: 3 Warnings: 0
mysql> select * from t1;
+------+
| a |
+------+
| 100 |
| 10 |
| 1 |
| 100 |
| 100 |
| 6 |
| 8 |
| 23 |
+------+
8 rows in set (0.00 sec)
mysql>
REPLACE
REPLACE [LOW_PRIORITY | DELAYED]
[INTO] tbl_name
[PARTITION (partition_name [, partition_name] ...)]
[(col_name [, col_name] ...)]
{VALUES | VALUE} (value_list) [, (value_list)] ...
REPLACE [LOW_PRIORITY | DELAYED]
[INTO] tbl_name
[PARTITION (partition_name [, partition_name] ...)]
SET assignment_list
REPLACE [LOW_PRIORITY | DELAYED]
[INTO] tbl_name
[PARTITION (partition_name [, partition_name] ...)]
[(col_name [, col_name] ...)]
SELECT ...
value:
{expr | DEFAULT}
value_list:
value [, value] ...
assignment:
col_name = value
assignment_list:
assignment [, assignment] ...
replace的原理是：先delete，在insert，如果没有替换对象时，类似于插入效果
操作
mysql> create table t4 ( a int auto_increment primary key, b int );
Query OK, 0 rows affected (0.03 sec)
mysql> insert into t4 values (null, 11);
Query OK, 1 row affected (0.02 sec)
mysql> insert into t4 values (null, 12);
Query OK, 1 row affected (0.00 sec)
mysql> insert into t4 values (null, 13);
Query OK, 1 row affected (0.01 sec)
mysql> select * from t4;
+---+------+
| a | b |
+---+------+
| 1 | 11 |
| 2 | 12 |
| 3 | 13 |
+---+------+
3 rows in set (0.00 sec)
mysql> replace into t4 values ( 1, 100);
Query OK, 2 rows affected (0.00 sec)
mysql> select * from t4;
+---+------+
| a | b |
+---+------+
| 1 | 100 |
| 2 | 12 |
| 3 | 13 |
+---+------+
3 rows in set (0.00 sec)
mysql> replace into t4 values (5, 200);
Query OK, 1 row affected (0.01 sec)
mysql> select * from t4;
+---+------+
| a | b |
+---+------+
| 1 | 100 |
| 2 | 12 |
| 3 | 13 |
| 5 | 200 |
+---+------+
4 rows in set (0.01 sec)
mysql>
mysql> create table t5 ( a int primary key, b int auto_increment, c int, key(b));
Query OK, 0 rows affected (0.03 sec)
mysql> insert into t5 values (10, null, 100);
Query OK, 1 row affected (0.01 sec)
mysql> insert into t5 values (20, null, 200);
Query OK, 1 row affected (0.01 sec)
mysql> select * from t5;
+----+---+------+
| a | b | c |
+----+---+------+
| 10 | 1 | 100 |
| 20 | 2 | 200 |
+----+---+------+
2 rows in set (0.00 sec)
mysql> replace into t5 values (10, null, 300);
Query OK, 2 rows affected (0.00 sec)
mysql> select * from t5;
+----+---+------+
| a | b | c |
+----+---+------+
| 10 | 3 | 300 |
| 20 | 2 | 200 |
+----+---+------+
2 rows in set (0.00 sec)
mysql>
insert on duplicate
mysql> select * from t4;
+---+------+
| a | b |
+---+------+
| 1 | 100 |
| 2 | 12 |
| 3 | 13 |
| 5 | 200 |
+---+------+
4 rows in set (0.00 sec)
mysql> show create table t4\G
*************************** 1. row ***************************
Table: t4
Create Table: CREATE TABLE `t4` (
`a` int(11) NOT NULL AUTO_INCREMENT,
`b` int(11) DEFAULT NULL,
PRIMARY KEY (`a`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8mb4
1 row in set (0.00 sec)
# 主键1 已存在，报错
mysql> insert into t4 values (1, 1);
ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'
# 带上 on duplicate参数，这是非sql标准，不推荐
mysql> insert into t4 values(1, 1) on duplicate key update b = 1;
Query OK, 2 rows affected (0.00 sec)
mysql> select * from t4;
+---+------+
| a | b |
+---+------+
| 1 | 1 |
| 2 | 12 |
| 3 | 13 |
| 5 | 200 |
+---+------+
4 rows in set (0.00 sec)
mysql>
INSERT IGNORE 忽略重复的错误
mysql> insert ignore t4 values(1, 1);
Query OK, 0 rows affected, 1 warning (0.00 sec)
mysql> show warnings;
+---------+------+---------------------------------------+
| Level | Code | Message |
+---------+------+---------------------------------------+
| Warning | 1062 | Duplicate entry '1' for key 'PRIMARY' |
+---------+------+---------------------------------------+
1 row in set (0.00 sec)
mysql>
其他操作
-
更新有关系的值
mysql> create table t6 ( a int, b int );
Query OK, 0 rows affected (0.03 sec)
mysql> insert into t6 values (1, 1);
Query OK, 1 row affected (0.00 sec)
mysql> select * from t6;
+------+------+
| a | b |
+------+------+
| 1 | 1 |
+------+------+
1 row in set (0.00 sec)
mysql> update t6 set a=a+1, b=a where a=1;
Query OK, 1 row affected (0.01 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from t6;
+------+------+
| a | b |
+------+------+
| 2 | 2 |
+------+------+
1 row in set (0.00 sec)
bgi
mysql>
UNION
- UNION 的作用是将两个查询的结果集进行合并。
- UNION必须由两条或两条以上的SELECT语句组成，语句之间用关键字UNION分隔。
- UNION中的每个查询必须包含相同的列（类型相同或可以隐式转换）、表达式或聚集函数。
mysql> create table test_union_1(a int, b int);
Query OK, 0 rows affected (0.18 sec)
mysql> create table test_union_2(a int, c int);
Query OK, 0 rows affected (0.15 sec)
mysql> insert into test_union_1 values(1, 2), (3, 4), (5, 6), (10, 20);
Query OK, 4 rows affected (0.06 sec)
Records: 4 Duplicates: 0 Warnings: 0
mysql> insert into test_union_2 values(10, 20), (30, 40), (50, 60);
Query OK, 3 rows affected (0.03 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> select * from test_union_1;
+------+------+
| a | b |
+------+------+
| 1 | 2 |
| 3 | 4 |
| 5 | 6 |
| 10 | 20 | -- test_union_1 中的10, 20
+------+------+
4 rows in set (0.00 sec)
mysql> select * from test_union_2;
+------+------+
| a | c |
+------+------+
| 10 | 20 | -- test_union_2 中的10, 20
| 30 | 40 |
| 50 | 60 |
+------+------+
3 rows in set (0.00 sec)
mysql> select a, b as t from test_union_1
-> union
-> select * from test_union_2;
+------+------+
| a | t |
+------+------+
| 1 | 2 |
| 3 | 4 |
| 5 | 6 |
| 10 | 20 | -- 只出现了一次 10, 20，union会去重
| 30 | 40 |
| 50 | 60 |
+------+------+
6 rows in set (0.00 sec)
mysql> select a, b as t from test_union_1
-> union all -- 使用 union all 可以不去重
-> select * from test_union_2;
+------+------+
| a | t |
+------+------+
| 1 | 2 |
| 3 | 4 |
| 5 | 6 |
| 10 | 20 | -- test_union_1 中的10, 20
| 10 | 20 | -- test_union_2 中的10, 20
| 30 | 40 |
| 50 | 60 |
+------+------+
7 rows in set (0.00 sec)
mysql> select a, b as t from test_union_1 where a > 2
-> union
-> select * from test_union_2 where c > 50; -- 使用where过滤也可以
+------+------+
| a | t |
+------+------+
| 3 | 4 |
| 5 | 6 |
| 10 | 20 |
| 50 | 60 |
+------+------+
4 rows in set (0.00 sec)