MySQL基礎之 如何刪除主鍵

我們在一個表中設置了主鍵之后，那么如何刪除主鍵呢？

刪除主鍵的語法是：

ALTER  TABLE  TABLE_NAME  DROP  PRIMARY  KEY;

在這里我們要考慮兩種情況：

1、可以直接使用drop刪除主鍵的情況。

mysql> create table test1_3(
    -> id int not null primary key,
    -> name char(10)
    -> );
Query OK, 0 rows affected (0.01 sec)

mysql> alter table test1_3 drop primary key;
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

2、如果帶有主鍵的列還有AUTO_INCREMENT屬性，需要間接方式去掉。

mysql> create table test1_2(
    -> id int not null  auto_increment,
    -> name char(10),-> primary key(id)
    -> );
Query OK, 0 rows affected (0.00 sec)

mysql> desc test1_2；+-------+----------+------+-----+---------+----------------+
| Field | Type     | Null | Key | Default | Extra          |
+-------+----------+------+-----+---------+----------------+
| id    | int(11)  | NO   | PRI | NULL    | auto_increment |
| name  | char(10) | YES  |     | NULL    |                |
+-------+----------+------+-----+---------+----------------+
2 rows in set (0.00 sec)

mysql> desc test1_2;
+-------+----------+------+-----+---------+----------------+
| Field | Type     | Null | Key | Default | Extra          |
+-------+----------+------+-----+---------+----------------+
| id    | int(11)  | NO   | PRI | NULL    | auto_increment |
| name  | char(10) | YES  |     | NULL    |                |
+-------+----------+------+-----+---------+----------------+
2 rows in set (0.00 sec)

mysql> alter table test1_2 drop primary key;
ERROR 1075 (42000): Incorrect table definition; there can be only one auto column and it must be defined as a key               #這說明此列是自動增長列，無法直接刪除
mysql> alter table test1_2 modify id int;
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> alter table test1_2 drop primary key;
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

所以說如果列的屬性還帶有AUTO_INCREMENT，那么要先將這個列的自動增長屬性去掉，才可以刪除主鍵。