MySQL学习之SQL基础（一）DDL

Sql基础

DDL （data defination language）

创建表

CREATE TABLE emp(
ename varchar(10),
hiredate date,
sal decimal(10,2),
deptno int(2)
);

mysql> show create table emp;
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

| Table | Create Table                                                                                                                                                                                                                                    |

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

| emp  | CREATE TABLE `emp` (

  `ename` varchar(10) COLLATE utf8_unicode_ci DEFAULT NULL,

  `hiredate` date DEFAULT NULL,

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

  `deptno` int(2) DEFAULT NULL

) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci |

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

1 row in set (0.01 sec)

修改表

• 修改表类型

ALTER TABLE emp  modify column ename varchar(20);

ALTER TABLE emp  modify  ename varchar(25);

• 增加表字段

ALTER TABLE emp add column age int (3);

• 删除表字段

ALTER TABLE  emp drop column age;

• 字段改名

ALTER TABLE emp change age age1 int(5);

• 修改顺序

mysql> ALTER TABLE emp add column age int (4) AFTER ename;
Query OK, 0 rows affected (0.06 sec)
Records: 0  Duplicates: 0  Warnings: 0
mysql> desc emp;
+----------+---------------+------+-----+---------+-------+

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

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

| ename    | varchar(25)  | YES  |    | NULL    |      |

| age      | int(4)        | YES  |    | NULL    |      |

| hiredate | date          | YES  |    | NULL    |      |

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

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

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