Shulou(Shulou.com)06/01 Report--

1. Mysql deletes the error of the primary key in the table, as shown in the following code:

Mysql > alter table student drop primary key;ERROR 1075 (42000): Incorrect table definition; there can be only one auto column and it must be defined as a key

2. The cause of the problem:

When you look at the type of the student table, you find that there is an auto_increment (incremental) type option in the primary key column. To delete the primary key in the table, you need to delete the auto_increment type first. The code is as follows:

Mysql > desc student +-+ | Field | Type | Null | Key | Default | Extra | + -+-+ | id | int (11) | NO | PRI | NULL | auto_increment | | name | char (20) | NO | MUL | NULL | | age | tinyint (2) | NO | MUL | 0 | | dept | varchar (16) | YES | NULL | | | | +-+-+ 4 rows in set (0.02 sec) |

3. Delete the auto_increment type above the primary key column in the student table. The code is as follows:

Mysql > alter table student change id id int;Query OK, 0 rows affected (0.04 sec) Records: 0 Duplicates: 0 Warnings: 0

Tip: the alter table student change id id int; command modifies the type of the column in the student table, and the auto_increment type is naturally deleted.

4. Check the type of the student table and find that the auto_increment type has been deleted. The code is as follows:

Mysql > desc student +-+ | Field | Type | Null | Key | Default | Extra | +-+- -+ | id | int (11) | NO | PRI | 0 | | name | char (20) | NO | MUL | NULL | | age | tinyint (2) | NO | MUL | 0 | | dept | varchar (16) | YES | | NULL | +-- -+ 4 rows in set (0.01 sec)

5. Delete the primary key in student. The code is as follows:

Mysql > alter table student drop primary key;Query OK, 0 rows affected (0.10 sec) Records: 0 Duplicates: 0 Warnings: 0

6. Finally, look at the type of student table and find that the primary key in the table has been deleted. The code is as follows:

Mysql > desc student +-+ | Field | Type | Null | Key | Default | Extra | +-+- -+ | id | int (11) | NO | | 0 | | name | char (20) | NO | MUL | NULL | | age | tinyint (2) | NO | MUL | 0 | | dept | varchar (16) | YES | | NULL | +- -+ 4 rows in set (0.00 sec)

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