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删除列 :
DROP COLUMN是用于从表中删除列。
语法是
ALTER TABLE tbl_name DROP col_name;
以下语句删除了标志的字段。
mysql> ALTER TABLE student DROP COLUMN marks;
Query OK, 0 rows affected (0.03 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc student;
+---------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| studid | int(10) | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
| address | varchar(40) | YES | | NULL | |
| phone | int(10) | YES | | NULL | |
+---------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)
更改列的名称:
当我们修改列的时，我们需要重新定义列的属性。以下的例子就是把学生表中的name字段重新定义为stud_name。
mysql> ALTER TABLE student CHANGE name stud_name VARCHAR(20);
Query OK, 0 rows affected (0.05 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc student;
+-----------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------+-------------+------+-----+---------+-------+
| studid | int(10) | YES | | NULL | |
| stud_name | varchar(20) | YES | | NULL | |
| marks | int(10) | YES | | NULL | |
| address | varchar(40) | YES | | NULL | |
| phone | int(10) | YES | | NULL | |
+-----------+-------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
更改列的名称:
如果我们想单独更改属性，我们可以使用相同的列，请参照以下的例子。
mysql> alter table student change name name varchar(40);
Query OK, 0 rows affected (0.05 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc student;
+---------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| studid | int(10) | YES | | NULL | |
| name | varchar(40) | YES | | NULL | |
| marks | int(10) | YES | | NULL | |
| address | varchar(40) | YES | | NULL | |
| phone | int(10) | YES | | NULL | |
+---------+-------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
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