MySQL基础二：数据表操作
参考该网站：http://www.imooc.com/video/2004
显示所有数据库：
mysql> SHOW DATABASES; +--------------------+ | Database | +--------------------+ | information_schema | | mysql | | performance_schema | | sakila | | sys | | t2 | | world | +--------------------+
打开一个数据库：
mysql> USE world Database changed显示当前打开的数据库：
mysql> SELECT DATABASE(); +------------+ | DATABASE() | +------------+ | world | +------------+ 1 row in set (0.00 sec)创建数据表：
mysql> CREATE TABLE IF NOT EXISTS tb1(
-> username VARCHAR(20),#字段，可变字符20字节、
-> age TINYINT UNSIGNED,#微型的整数、
-> salary FLOAT(8,2) UNSIGNED #浮点型，总共八位，小数两位、
-> );
Query OK, 0 rows affected (0.32 sec)查看数据表：
mysql> SHOW TABLES
-> ;
+-----------------+
| Tables_in_world |
+-----------------+
| city |
| country |
| countrylanguage |
| tb1 |
+-----------------+
4 rows in set (0.01 sec)查看其他数据库的数据表：
mysql> SHOW TABLES FROM mysql
-> ;
+---------------------------+
| Tables_in_mysql |
+---------------------------+
| columns_priv |
| db |
| engine_cost |
| event |
| func |
| general_log |
| gtid_executed |
| help_category |
| help_keyword |
| help_relation |
| help_topic |
| innodb_index_stats |
| innodb_table_stats |
| ndb_binlog_index |
| plugin |
| proc |
| procs_priv |
| proxies_priv |
| server_cost |
| servers |
| slave_master_info |
| slave_relay_log_info |
| slave_worker_info |
| slow_log |
| tables_priv |
| time_zone |
| time_zone_leap_second |
| time_zone_name |
| time_zone_transition |
| time_zone_transition_type |
| user |
+---------------------------+
31 rows in set (0.00 sec)
查看数据表结构：
mysql> SHOW COLUMNS FROM tb1; +----------+---------------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +----------+---------------------+------+-----+---------+-------+ | username | varchar(20) | YES | | NULL | | | age | tinyint(3) unsigned | YES | | NULL | | | salary | float(8,2) unsigned | YES | | NULL | | +----------+---------------------+------+-----+---------+-------+ 3 rows in set (0.03 sec)
向数据表中写入记录：
mysql> INSERT INTO tb1 VALUES('tom',25,7863.25);#其中INTO可以省略
Query OK, 1 row affected (0.17 sec)第二种写入方法，不是所有字段都赋值；
mysql> INSERT tb1(username,salary) VALUES('John',4500.69);
Query OK, 1 row affected (0.11 sec)
记录查找用select语句，*表示字段的过滤：
mysql> SELECT * FROM tb1; +----------+------+---------+ | username | age | salary | +----------+------+---------+ | tom | 25 | 7863.25 | | John | NULL | 4500.69 | +----------+------+---------+ 2 rows in set (0.02 sec)
NULL字段是可以为空，可缺省；NOT NULL 是不可缺省：
mysql> CREATE TABLE tb2(
-> username VARCHAR(20) NOT NULL,
-> age TINYINT UNSIGNED NULL #其中NULL可以省略
-> );
Query OK, 0 rows affected (0.33 sec)
mysql> SHOW COLUMNS FROM tb2;
+----------+---------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+---------------------+------+-----+---------+-------+
| username | varchar(20) | NO | | NULL | |
| age | tinyint(3) unsigned | YES | | NULL | |
+----------+---------------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
mysql> INSERT tb2 VALUES('TOM',NULL);
Query OK, 1 row affected (0.04 sec)
mysql> SELECT * FROM tb2;
+----------+------+
| username | age |
+----------+------+
| TOM | NULL |
+----------+------+
1 row in set (0.00 sec)
mysql> INSERT tb2 VALUES(NULL,26);
ERROR 1048 (23000): Column 'username' cannot be null
mysql> CREATE TABLE tb3(
-> id SMALLINT UNSIGNED AUTO_INCREMENT,
-> username VARCHAR(30) NOT NULL
-> );
ERROR 1075 (42000): Incorrect table definition; there can be only one auto column and it must be defined as a key创建带主键的自动编号表：
<pre name="code" class="sql">mysql> CREATE TABLE tb3(
-> id SMALLINT UNSIGNED AUTO_INCREMENT,
-> username VARCHAR(30) NOT NULL
-> );
ERROR 1075 (42000): Incorrect table definition; there can be only one auto column and it must be defined as a key
mysql> CREATE TABLE tb3(
-> id SMALLINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
-> username VARCHAR(30) NOT NULL
-> );
Query OK, 0 rows affected (0.20 sec)
mysql> SHOW COLUMNS FROM tb3;
+----------+----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------+----------------------+------+-----+---------+----------------+
| id | smallint(5) unsigned | NO | PRI | NULL | auto_increment |
| username | varchar(30) | NO | | NULL | |
+----------+----------------------+------+-----+---------+----------------+
2 rows in set (0.03 sec)
mysql> INSERT tb3(username) VALUES('Tom');
Query OK, 1 row affected (0.05 sec)
mysql> INSERT tb3(username) VALUES('John');
Query OK, 1 row affected (0.06 sec)
mysql> INSERT tb3(username) VALUES('Rose');
Query OK, 1 row affected (0.05 sec)
mysql> INSERT tb3(username) VALUES('Dimitar');
Query OK, 1 row affected (0.05 sec)
mysql> SELECT * FROM tb3;
+----+----------+
| id | username |
+----+----------+
| 1 | Tom |
| 2 | John |
| 3 | Rose |
| 4 | Dimitar |
+----+----------+
4 rows in set (0.00 sec)
mysql> CREATE TABLE tb4(
-> id SMALLINT UNSIGNED PRIMARY KEY,
-> username VARCHAR(20) NOT NULL
-> );
Query OK, 0 rows affected (0.19 sec)
主键可以没有AUTO_INCRMENT：
mysql> SHOW COLUMNS FROM tb4; +----------+----------------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +----------+----------------------+------+-----+---------+-------+ | id | smallint(5) unsigned | NO | PRI | NULL | | | username | varchar(20) | NO | | NULL | | +----------+----------------------+------+-----+---------+-------+ 2 rows in set (0.00 sec) mysql> INSERT tb4 VALUES(4,'Tom'); Query OK, 1 row affected (0.06 sec) mysql> INSERT tb4 VALUES(22,'John'); Query OK, 1 row affected (0.21 sec) mysql> SELECT * FROM tb4; +----+----------+ | id | username | +----+----------+ | 4 | Tom | | 22 | John | +----+----------+ 2 rows in set (0.00 sec) mysql> INSERT tb4 VALUES(22,'Rose'); ERROR 1062 (23000): Duplicate entry '22' for key 'PRIMARY'
另外一个唯一约束，unique约束，创建一个既有主键又有唯一的表：
mysql> CREATE TABLE tb5(
-> id SMALLINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
-> username VARCHAR(20) NOT NULL UNIQUE KEY,
-> age TINYINT UNSIGNED
-> );
Query OK, 0 rows affected (0.22 sec)
mysql>
mysql> SHOW COLUMNS FROM tb5;
+----------+----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------+----------------------+------+-----+---------+----------------+
| id | smallint(5) unsigned | NO | PRI | NULL | auto_increment |
| username | varchar(20) | NO | UNI | NULL | |
| age | tinyint(3) unsigned | YES | | NULL | |
+----------+----------------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)
mysql> INSERT tb5(username,age) VALUES('Tom',22);
Query OK, 1 row affected (0.04 sec)
mysql> INSERT tb5(username,age) VALUES('Tom',22);
ERROR 1062 (23000): Duplicate entry 'Tom' for key 'username'
默认约束：
mysql> CREATE TABLE tb6(
-> id SMALLINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
-> username VARCHAR(20) NOT NULL UNIQUE KEY,
-> sex ENUM('1','2','3') DEFAULT '3'
-> );
Query OK, 0 rows affected (0.25 sec)
mysql> SHOW COLUMNS FROM tb6;
+----------+----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------+----------------------+------+-----+---------+----------------+
| id | smallint(5) unsigned | NO | PRI | NULL | auto_increment |
| username | varchar(20) | NO | UNI | NULL | |
| sex | enum('1','2','3') | YES | | 3 | |
+----------+----------------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)
mysql> SELECT * FROM tb6;
Empty set (0.00 sec)