mysql数据库——修改表结构

修改表结构

1 问题

本案例要求熟悉MySQL库中表的字段修改，主要练习以下操作：
• 添加字段
• 修改字段名
• 修改字段类型
• 删除字段

2 步骤

实现此案例需要按照如下步骤进行。
步骤一：添加字段
在studb中创建tea6表

mysql> CREATE TABLE studb.tea6(id int(4) PRIMARY KEY,
    -> name varchar(4) NOT NULL,
    -> age int(2) NOT NULL
    -> );
Query OK, 0 rows affected (0.34 sec)

为tea6表添加一个address字段
添加前：

mysql> DESC tea6;
+-------+------------+------+-----+---------+-------+
| Field | Type       | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| id    | int(4)     | NO   | PRI | NULL    |       |
| name  | varchar(4) | NO   |     | NULL    |       |
| age   | int(2)     | NO   |     | NULL    |       |
+-------+------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

添加address字段

mysql> ALTER TABLE tea6 ADD address varchar(48);
Query OK, 0 rows affected (0.84 sec)
Records: 0  Duplicates: 0  Warnings: 0

添加后，默认最后一个字段

mysql> DESC tea6;
+---------+-------------+------+-----+---------+-------+
| Field   | Type        | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| id      | int(4)      | NO   | PRI | NULL    |       |
| name    | varchar(4)  | NO   |     | NULL    |       |
| age     | int(2)      | NO   |     | NULL    |       |
| address | varchar(48) | YES  |     | NULL    |       |
+---------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

在tea6表的age列之后添加一个gender字段
添加操作：

mysql> ALTER TABLE tea6 ADD gender enum('boy','girl') AFTER age;
Query OK, 0 rows affected (0.59 sec)
Records: 0  Duplicates: 0  Warnings: 0

确认添加结果

mysql> DESC tea6;
+---------+--------------------+------+-----+---------+-------+
| Field   | Type               | Null | Key | Default | Extra |
+---------+--------------------+------+-----+---------+-------+
| id      | int(4)             | NO   | PRI | NULL    |       |
| name    | varchar(4)         | NO   |     | NULL    |       |
| age     | int(2)             | NO   |     | NULL    |       |gender  | enum('boy','girl') | YES  |     | NULL    |       |
| address | varchar(48)        | YES  |     | NULL    |       |
+---------+--------------------+------+-----+---------+-------+
5 rows in set (0.00 sec)

步骤二：修改字段名和字段类型
将tea6表的gender字段改名为sex，并添加非空约束
修改操作：

mysql> ALTER TABLE tea6 CHANGE gender
   -> sex enum('boy','girl') NOT NULL;
 Query OK, 0 rows affected (0.08 sec)
 Records: 0  Duplicates: 0  Warnings: 0

确认修改结果

mysql> DESC tea6;
+---------+--------------------+------+-----+---------+-------+
| Field   | Type               | Null | Key | Default | Extra |
+---------+--------------------+------+-----+---------+-------+
| id      | int(4)             | NO   | PRI | NULL    |       |
| name    | varchar(4)         | NO   |     | NULL    |       |
| age     | int(2)             | NO   |     | NULL    |       |
| sex     | enum('boy','girl') | NO   |     | NULL    |       |
| address | varchar(48)        | YES  |     | NULL    |       |
+---------+--------------------+------+-----+---------+-------+
5 rows in set (0.00 sec)

步骤三：删除字段
删除tea6表中名为sex的字段：

mysql> ALTER TABLE tea6 DROP sex;                             //删除操作
Query OK, 0 rows affected (0.52 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> DESC tea6;                                          //确认删除结果
+---------+-------------+------+-----+---------+-------+
| Field   | Type        | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| id      | int(4)      | NO   | PRI | NULL    |       |
| name    | varchar(4)  | NO   |     | NULL    |       |
| age     | int(2)      | NO   |     | NULL    |       |
| address | varchar(48) | YES  |     | NULL    |       |
+---------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

本文地址：https://blog.csdn.net/weixin_47062057/article/details/107639337