标签：指定 ISE creat varchar show 3.2 16px 设置 extra
ALTER命令：
作用：当我们修改数据库的列属性、列名称、表名等，要使用ALTER命令
教程：
1、首先是我们创建一个数据库以及一张表
mysql> create table exercise( -> id int, -> name varchar(15) -> );
1.1、查看表结构
mysql> SHOW COLUMnS FROM exercise; 或者使用 DESC erercise； +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(15) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 2 rows in set (0.00 sec)
删除、添加、修改表字段
1、删除一个字段，比如id
mysql> ALTER TABLE exercise drop id; Query OK, 0 rows affected (0.01 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> DESC exercise; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | name | varchar(15) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 1 row in set (0.00 sec)
2、添加字段
2.1、添加一个字段，比如id_new
mysql> ALTER TABLE exercise ADD COLUMN id_new int; Query OK, 0 rows affected (0.01 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> DESC exercise; +--------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------+-------------+------+-----+---------+-------+ | name | varchar(15) | YES | | NULL | | | id_new | int(11) | YES | | NULL | | +--------+-------------+------+-----+---------+-------+ 2 rows in set (0.00 sec)
2.2、将新添加的字段设置为第一列
mysql> ALTER TABLE exercise ADD COLUMN sex varchar(1) FIRST; Query OK, 0 rows affected (0.02 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> DESC exercise; +--------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------+-------------+------+-----+---------+-------+ | sex | varchar(1) | YES | | NULL | | | name | varchar(15) | YES | | NULL | | | id_new | int(11) | YES | | NULL | | +--------+-------------+------+-----+---------+-------+ 3 rows in set (0.00 sec)
2.3、在某个列之后添加新的字段，比如在name列后添加
mysql> ALTER TABLE exercise ADD COLUMN address char(50) AFTER name; Query OK, 0 rows affected (0.01 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> DESC exercise; +---------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+-------------+------+-----+---------+-------+ | sex | varchar(1) | YES | | NULL | | | name | varchar(15) | YES | | NULL | | | address | char(50) | YES | | NULL | | | id_new | int(11) | YES | | NULL | | +---------+-------------+------+-----+---------+-------+ 4 rows in set (0.00 sec)
总结：添加字段，只有FIRST和AFTER关键字两个
3、修改一个字段属性及名称
3.1、修改列属性
使用modify修改列属性
mysql> ALTER TABLE exercise modify id_new int(4); #将原来的int（11）修改为int（4） Query OK, 0 rows affected (0.00 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> DESC exercise; +--------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------+-------------+------+-----+---------+-------+ | name | varchar(15) | YES | | NULL | | | id_new | int(4) | YES | | NULL | | +--------+-------------+------+-----+---------+-------+ 2 rows in set (0.00 sec)
使用change修改列属性
mysql> ALTER TABLE exercise CHANGE id_new id_new int(8); Query OK, 0 rows affected (0.01 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> DESC exercise; +--------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------+-------------+------+-----+---------+-------+ | name | varchar(15) | YES | | NULL | | | id_new | int(8) | YES | | NULL | | +--------+-------------+------+-----+---------+-------+ 2 rows in set (0.00 sec)
总结：modify和change的区别：修改列属性时，使用change比较麻烦，要写两次列名称，而使用modify只需要写一次列名称就可以了。
3.2、修改列名字
mysql> ALTER TABLE exercise CHANGE id_new id_new_new; ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ‘‘ at line 1 mysql> ALTER TABLE exercise CHANGE id_new id_new_new int(8); Query OK, 0 rows affected (0.00 sec) Records: 0 Duplicates: 0 Warnings: 0
mysql> DESC exercise;
+------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| name | varchar(15) | YES | | NULL | |
| id_new_new | int(8) | YES | | NULL | |
+------------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
总结：修改列名称需要使用CHANGE，而MODIFY不能修改列的名称。如果要同时修改名称和属性还是CHANGE。使用CHANGE后面要跟上旧列、新列的名称，以及列属性。因此修改名称前先查看列属性。
一句话：字段名称CHANGE；字段属性MODIFY。即名称又属性还是CHANGE
4、修改表名称
mysql> ALTER TABLE exercise rename exercise_new; Query OK, 0 rows affected (0.01 sec) mysql> show tables; +----------------+ | Tables_in_test | +----------------+ | exercise_new | +----------------+ 1 row in set (0.00 sec)
5、ALTER 修改字段的默认值
我们往往在建表的时候给某个列的属性上面设置默认值，比如“DEFAULT NULL”等等。如何修改默认值呢？首先来查看所有列的默认值
mysql> SHOW COLUMNS FROM exercise; +---------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+-------------+------+-----+---------+-------+ | sex | varchar(1) | YES | | NULL | | | name | varchar(15) | YES | | NULL | | | address | char(50) | YES | | NULL | | | id_new | int(11) | YES | | NULL | | +---------+-------------+------+-----+---------+-------+ 4 rows in set (0.00 sec)
基本所有的列默认值DEFAULT都是NULL，现在我们指定“sex”列默认值是“M”
mysql> ALTER TABLE exercise ALTER sex SET DEFAULT ‘M‘; Query OK, 0 rows affected (0.01 sec) Records: 0 Duplicates: 0 Warnings: 0
查看一下：
mysql> SHOW COLUMNS FROM exercise; +---------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+-------------+------+-----+---------+-------+ | sex | varchar(1) | YES | | M | | | name | varchar(15) | YES | | NULL | | | address | char(50) | YES | | NULL | | | id_new | int(11) | YES | | NULL | | +---------+-------------+------+-----+---------+-------+ 4 rows in set (0.00 sec)
尝试删除表中一个列的默认值
mysql> ALTER TABLE exercise ALTER sex drop DEFAULT; Query OK, 0 rows affected (0.00 sec) Records: 0 Duplicates: 0 Warnings: 0
再来查看一下：
mysql> SHOW COLUMNS FROM exercise; +---------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+-------------+------+-----+---------+-------+ | sex | varchar(1) | YES | | NULL | | | name | varchar(15) | YES | | NULL | | | address | char(50) | YES | | NULL | | | id_new | int(11) | YES | | NULL | | +---------+-------------+------+-----+---------+-------+ 4 rows in set (0.00 sec)
标签：指定 ISE creat varchar show 3.2 16px 设置 extra
原文地址：https://www.cnblogs.com/FengGeBlog/p/9771646.html
