mysql为已经建立的表设置其主键（primary key）

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Database changed
mysql> desc orders;
+-------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------------+-------------+------+-----+---------+-------+
| Company | varchar(20) | YES | | NULL | |
| OrderNumber | int(5) | YES | | NULL | |
| Id_O | int(4) | NO | | 1 | |

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

建立的orders如上，现在想设置Id_O为primary key

应该先将其删除

mysql > alter table orders drop Id_O;

在填曾：

mysql> alter table Orders add Id_O int not null primary key Auto_increment; ;

需要设置其为自增型，否则，因为空缺等问题而不能设置Id_O为主键；

mysql> select * from orders;
+---------- +------------- +----+
| Company | OrderNumber | Id |
+---------- +----------- --+----+
| IBM | 3532 | 1 |
| W3School | 2356 | 2 |
| Apple | 4698 | 3 |
| W3School | 6953 | 4 |
+---------- +------------- +----+

改变primary key为自增型：

alter table tb_name modify id int auto_increment.

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