mysql的子查询及视图
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5.1 子查询 — 在一个 select 语句中，嵌入另一个 select 语句，那么被嵌入的 select 语句被称之为子查询语句。
CURD -- 增删改查
添加 insert into / create
修改 update / alter
读取 select / show
删除 delete / drop
==》select 查询：
1）条件查询（比较查询，范围查询，模糊查询，逻辑查询）
2）连接查询（内联，左联，右联，自联/自查询；）
3）子查询
==》关系：子查询是主查询辅助查询，充当条件，要么当作数据源，要么当作值
==》子查询分类：
标量子查询：一个数据
列子查询：返回一列
行子查询：返回一行
1、标量子查询：子查询的结果返回一个值，作为主查询的条件
```
MariaDB [shop]> select * from areas where pid=(select id from areas where area='广州市');
+-----+--------+------+
| id | area | pid |
+-----+--------+------+
| 230 | 越秀区 | 98 |
| 231 | 荔湾区 | 98 |
| 232 | 海珠区 | 98 |
| 233 | 天河区 | 98 |
| 234 | 白云区 | 98 |
| 235 | 黄埔区 | 98 |
| 236 | 番禺区 | 98 |
| 237 | 花都区 | 98 |
| 238 | 南沙区 | 98 |
| 239 | 增城区 | 98 |
| 240 | 从化区 | 98 |
+-----+--------+------+
11 rows in set (0.00 sec)
MariaDB [shop]> select * from student where cls_id=(select id from class where teacher='孙静香');
+----+--------+--------+
| id | sname | cls_id |
+----+--------+--------+
| 1 | 谢逊 | 1 |
| 2 | 周大福 | 1 |
| 3 | 高程远 | 1 |
+----+--------+--------+
3 rows in set (0.00 sec)
MariaDB [shop]> select count(id) from student where cls_id=(select id from class where teacher='孙静香');
+-----------+
| count(id) |
+-----------+
| 3 |
+-----------+
1 row in set (0.00 sec)
```
2、列子查询：子查询返回一个字段下的所有或者多个值或者一个值，作为主查询条件
```
MariaDB [shop]> update student set cls_id=(select id from class where teacher='王三多') where sname='谢三逊';
Query OK, 1 row affected (0.01 sec)
Rows matched: 1 Changed: 1 Warnings: 0
MariaDB [shop]> select * from student;
+----+--------+--------+
| id | sname | cls_id |
+----+--------+--------+
| 1 | 谢逊 | 1 |
| 2 | 周大福 | 1 |
| 3 | 高程远 | 1 |
| 4 | 谢三逊 | 2 |
+----+--------+--------+
4 rows in set (0.00 sec)
MariaDB [shop]> select * from student where cls_id in (select id from class where teacher='孙静香' or teacher='王三多');
+----+--------+--------+
| id | sname | cls_id |
+----+--------+--------+
| 1 | 谢逊 | 1 |
| 2 | 周大福 | 1 |
| 3 | 高程远 | 1 |
| 4 | 谢三逊 | 2 |
+----+--------+--------+
4 rows in set (0.00 sec)
MariaDB [shop]> select id from class where teacher='孙静香' or teacher='王三多';
+----+
| id |
+----+
| 1 |
| 2 |
+----+
2 rows in set (0.00 sec)
```
3、行子查询
```
MariaDB [shop]> create table xs(
-> id int primary key auto_increment,
-> sname varchar(20),
-> height float(3,2),
-> age tinyint
-> )engine=innodb default charset=utf8;
Query OK, 0 rows affected (0.02 sec)
MariaDB [shop]> desc xs;
+--------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| sname | varchar(20) | YES | | NULL | |
| height | float(3,2) | YES | | NULL | |
| age | tinyint(4) | YES | | NULL | |
+--------+-------------+------+-----+---------+----------------+
4 rows in set (0.01 sec)
MariaDB [shop]> insert into xs (sname,height,age)values('吴亦凡',1.25,10),('陈兴',1.50,27),('张奇',1.40,12);
Query OK, 3 rows affected (0.00 sec)
Records: 3 Duplicates: 0 Warnings: 0
MariaDB [shop]> select max(height),max(age) from xs;
+-------------+----------+
| max(height) | max(age) |
+-------------+----------+
| 1.50 | 27 |
+-------------+----------+
1 row in set (0.00 sec)
MariaDB [shop]> select * from xs where (height,age)=(select max(height),max(age) from xs);
+----+-------+--------+------+
| id | sname | height | age |
+----+-------+--------+------+
| 2 | 陈兴 | 1.50 | 27 |
+----+-------+--------+------+
1 row in set (0.00 sec)
```
总结：有多步查询，子查询的结果作为主查询的条件，或者数据；
在列查询里 in ：主查询 where 条件 in （子查询）
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5.2 视图 — 一张虚拟表
概念：通俗来讲，视图就是一条select查询的结果集
命令格式：create view 视图名字 as select语句;
名称建议：v_xxx
==》案例：
```
MariaDB [books]> create view v_stu as select s.id as id, s.name as name, jsj.c_name as jsj,yy.c_name as yy from student s left join score jsj on s.id=jsj.stu_id left join score yy on s.id=yy.stu_id where jsj.c_name=‘计算机’ and yy.c_name=‘英语’;
Query OK, 0 rows affected (0.01 sec)
MariaDB [books]> show tables;
+-----------------+
| Tables_in_books |
+-----------------+
| books |
| score |
| student |
| v_stu |
+-----------------+
4 rows in set (0.00 sec)
```
==》查看视图：
show tables;
==》使用视图：
select * from 视图名;
==》删除视图：
drop view 视图名;
==》修改视图：
create or replace view 视图名字 as select语句;
```
MariaDB [books]> create or replace view v_stu as select * from student;
Query OK, 0 rows affected (0.01 sec)
MariaDB [books]> show tables;
+-----------------+
| Tables_in_books |
+-----------------+
| books |
| score |
| student |
| v_stu |
+-----------------+
4 rows in set (0.00 sec)
```
==》视图作用：
提高了重用性；
可以对数据重构，但不影响数据库结构；
提高了安全性能，让数据更清晰
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外键（同样涉及 CURD-增删改查）
==》在创建表时，添加外键：
```
CREATE TABLE dingdan (
id int(11) NOT NULL AUTO_INCREMENT COMMENT ‘订单编号’,
title varchar(50) DEFAULT NULL COMMENT ‘订单标题’,
add_time datetime DEFAULT NULL COMMENT ‘下单时间’,
user int(11) DEFAULT NULL COMMENT ‘用户编号’,
status enum(‘待付款’,‘待发货’,‘待收货’,‘已退单’,‘已完成’) DEFAULT NULL,
PRIMARY KEY (id)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8;
create table t(
id int primary key auto_increment comment '主键 ',
name varchar(20) comment '测试名字',
oid int,
foreign key(oid) references dingdan(id)
);
insert into dingdan (title,add_time,user,status)values('购买苹果手机','2019-04-01 12:00:00', '张三',3);
```
==》创建表时，添加外键的语法：
foreign key(外键字段) references 外部表名字(外部表主键)
==》创建表后，添加外键的语法（不指定外键名称）：
alter table 表名 add foreign key(本表外键字段) references 外部表名(外部表主键);
==》创建表后，添加外键的语法（指定外键名称）：
alter table 表名 add constraint 外键名 foreign key(本表外键字段) references 外部表名(外部表主键);
==》删除外键：
MariaDB [shop]> alter table t drop foreign key t_ibfk_1; Query OK, 0 rows affected (0.01 sec) Records: 0 Duplicates: 0 Warnings: 0
注意：外键的名字，不一定是字段的名字，可以用 show create table 表名 来查看
pid : parent id ->pid
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