mysql实例之使用alter table命令修改表结构

实例一：使用ALTER TABLE命令向表中添加字段、修改字段类型以及设置主键。

首先创建一个表，SQL语句如下：

mysql> CREATE TABLE myTable(
    ->    ID SMALLINT
    -> );

使用desc命令查看表结构：

mysql> desc myTable;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| ID    | smallint(6) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+

使用ALTER TABLE命令向表中添加字段、修改字段类型以及设置主键，SQL语句如下：

mysql> ALTER TABLE myTable
    ->    ADD COLUMN Quantity SMALLINT UNSIGNED NOT NULL,
    ->    MODIFY ID SMALLINT UNSIGNED NOT NULL,
    ->    ADD PRIMARY KEY (ID);

再次使用desc命令查看表结构，我们发现上面的修改命令生效了：

desc myTable;
+----------+----------------------+------+-----+---------+-------+
| Field    | Type                 | Null | Key | Default | Extra |
+----------+----------------------+------+-----+---------+-------+
| ID       | smallint(5) unsigned | NO   | PRI |         |       |
| Quantity | smallint(5) unsigned | NO   |     |         |       |
+----------+----------------------+------+-----+---------+-------+

实例二：在上面的实例基础上，我们删除myTable表中的Quantity字段和PRIMARY KEY主键

mysql> ALTER TABLE myTable
    ->    DROP COLUMN Quantity,
    ->    DROP PRIMARY KEY;

查看表结构：

mysql> desc myTable;
+-------+----------------------+------+-----+---------+-------+
| Field | Type                 | Null | Key | Default | Extra |
+-------+----------------------+------+-----+---------+-------+
| ID    | smallint(5) unsigned | NO   |     |         |       |
+-------+----------------------+------+-----+---------+-------+