MySQL-SQL基础

mysql> use test;

Database changed

mysql> create table emp(ename varchar(10),hirdate date,sal decimal(10,2),deptno int(2));

Query OK, 0 rows affected (0.01 sec)

mysql> show tables;

+----------------+

| Tables_in_test |

+----------------+

| emp            |

+----------------+

1 row in set (0.00 sec)

mysql> desc emp;

+---------+---------------+------+-----+---------+-------+

| Field   | Type          | Null | Key | Default | Extra |

+---------+---------------+------+-----+---------+-------+

| ename   | varchar(10)   | YES  |     | NULL    |       |

| hirdate | date          | YES  |     | NULL    |       |

| sal     | decimal(10,2) | YES  |     | NULL    |       |

| deptno  | int(2)        | YES  |     | NULL    |       |

+---------+---------------+------+-----+---------+-------+

4 rows in set (0.00 sec)

mysql> show create table emp\G

*************************** 1. row ***************************

       Table: emp

Create Table: CREATE TABLE `emp` (

  `ename` varchar(10) DEFAULT NULL,

  `hirdate` date DEFAULT NULL,

  `sal` decimal(10,2) DEFAULT NULL,

  `deptno` int(2) DEFAULT NULL

) ENGINE=InnoDB DEFAULT CHARSET=utf8

1 row in set (0.00 sec)

mysql> alter table emp modify ename varchar(20);

Query OK, 0 rows affected (0.01 sec)

Records: 0  Duplicates: 0  Warnings: 0

mysql> desc emp;

+---------+---------------+------+-----+---------+-------+

| Field   | Type          | Null | Key | Default | Extra |

+---------+---------------+------+-----+---------+-------+

| ename   | varchar(20)   | YES  |     | NULL    |       |

| hirdate | date          | YES  |     | NULL    |       |

| sal     | decimal(10,2) | YES  |     | NULL    |       |

| deptno  | int(2)        | YES  |     | NULL    |       |

+---------+---------------+------+-----+---------+-------+

4 rows in set (0.00 sec)

mysql> alter table emp add column age int(3);

Query OK, 0 rows affected (0.02 sec)

Records: 0  Duplicates: 0  Warnings: 0

mysql> desc emp;

+---------+---------------+------+-----+---------+-------+

| Field   | Type          | Null | Key | Default | Extra |

+---------+---------------+------+-----+---------+-------+

| ename   | varchar(20)   | YES  |     | NULL    |       |

| hirdate | date          | YES  |     | NULL    |       |

| sal     | decimal(10,2) | YES  |     | NULL    |       |

| deptno  | int(2)        | YES  |     | NULL    |       |

| age     | int(3)        | YES  |     | NULL    |       |

+---------+---------------+------+-----+---------+-------+

5 rows in set (0.00 sec)

mysql> alter table emp drop column age;

Query OK, 0 rows affected (0.02 sec)

Records: 0  Duplicates: 0  Warnings: 0

mysql> desc emp;

+---------+---------------+------+-----+---------+-------+

| Field   | Type          | Null | Key | Default | Extra |

+---------+---------------+------+-----+---------+-------+

| ename   | varchar(20)   | YES  |     | NULL    |       |

| hirdate | date          | YES  |     | NULL    |       |

| sal     | decimal(10,2) | YES  |     | NULL    |       |

| deptno  | int(2)        | YES  |     | NULL    |       |

+---------+---------------+------+-----+---------+-------+

4 rows in set (0.00 sec)

mysql> alter table emp add column age int(3);

Query OK, 0 rows affected (0.01 sec)

Records: 0  Duplicates: 0  Warnings: 0

mysql> alter table emp change column age age1 int(4);

Query OK, 0 rows affected (0.01 sec)

Records: 0  Duplicates: 0  Warnings: 0

mysql> desc emp;

+---------+---------------+------+-----+---------+-------+

| Field   | Type          | Null | Key | Default | Extra |

+---------+---------------+------+-----+---------+-------+

| ename   | varchar(20)   | YES  |     | NULL    |       |

| hirdate | date          | YES  |     | NULL    |       |

| sal     | decimal(10,2) | YES  |     | NULL    |       |

| deptno  | int(2)        | YES  |     | NULL    |       |

| age1    | int(4)        | YES  |     | NULL    |       |

+---------+---------------+------+-----+---------+-------+

5 rows in set (0.00 sec)

mysql> alter table emp add birth date after ename;

Query OK, 0 rows affected (0.02 sec)

Records: 0  Duplicates: 0  Warnings: 0

mysql>

mysql> desc emp;

+---------+---------------+------+-----+---------+-------+

| Field   | Type          | Null | Key | Default | Extra |

+---------+---------------+------+-----+---------+-------+

| ename   | varchar(20)   | YES  |     | NULL    |       |

| birth   | date          | YES  |     | NULL    |       |

| hirdate | date          | YES  |     | NULL    |       |

| sal     | decimal(10,2) | YES  |     | NULL    |       |

| deptno  | int(2)        | YES  |     | NULL    |       |

| age1    | int(4)        | YES  |     | NULL    |       |

+---------+---------------+------+-----+---------+-------+

6 rows in set (0.00 sec)

mysql> alter table emp rename emp1;

Query OK, 0 rows affected (0.01 sec)

mysql> desc emp1;

+---------+---------------+------+-----+---------+-------+

| Field   | Type          | Null | Key | Default | Extra |

+---------+---------------+------+-----+---------+-------+

| ename   | varchar(20)   | YES  |     | NULL    |       |

| birth   | date          | YES  |     | NULL    |       |

| hirdate | date          | YES  |     | NULL    |       |

| sal     | decimal(10,2) | YES  |     | NULL    |       |

| deptno  | int(2)        | YES  |     | NULL    |       |

| age1    | int(4)        | YES  |     | NULL    |       |

+---------+---------------+------+-----+---------+-------+

6 rows in set (0.00 sec)

mysql>