添加单列
ALTER TABLE tbl_name ADD [COLUMN] col_name column_definition [FIRST|AFTER col_name]
mysql> show columns from user; +----------+-------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +----------+-------------+------+-----+---------+----------------+ | id | smallint(6) | NO | PRI | NULL | auto_increment | | username | varchar(20) | YES | | NULL | | | pid | smallint(6) | YES | MUL | NULL | | +----------+-------------+------+-----+---------+----------------+ 3 rows in set mysql> describe user; +----------+-------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +----------+-------------+------+-----+---------+----------------+ | id | smallint(6) | NO | PRI | NULL | auto_increment | | username | varchar(20) | YES | | NULL | | | pid | smallint(6) | YES | MUL | NULL | | +----------+-------------+------+-----+---------+----------------+ 3 rows in set
添加一列
mysql> alter table user add userno varchar(20) not null; Database changed mysql> describe user; +----------+-------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +----------+-------------+------+-----+---------+----------------+ | id | smallint(6) | NO | PRI | NULL | auto_increment | | username | varchar(20) | YES | | NULL | | | pid | smallint(6) | YES | MUL | NULL | | | userno | varchar(20) | NO | | | | +----------+-------------+------+-----+---------+----------------+ 4 rows in set
age 到所有列前面
mysql> alter table user add age varchar(2) first; Database changed Records: 1 Duplicates: 0 Warnings: 0 mysql> describe user; +----------+-------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +----------+-------------+------+-----+---------+----------------+ | age | varchar(2) | YES | | NULL | | | id | smallint(6) | NO | PRI | NULL | auto_increment | | username | varchar(20) | YES | | NULL | | | pid | smallint(6) | YES | MUL | NULL | | | userno | varchar(20) | NO | | | | +----------+-------------+------+-----+---------+----------------+ 5 rows in set
sex 到username后面
mysql> alter table user add column sex char(1) after username; Database changed Records: 1 Duplicates: 0 Warnings: 0 mysql> describe user; +----------+-------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +----------+-------------+------+-----+---------+----------------+ | age | varchar(2) | YES | | NULL | | | id | smallint(6) | NO | PRI | NULL | auto_increment | | username | varchar(20) | YES | | NULL | | | sex | char(1) | YES | | NULL | | | pid | smallint(6) | YES | MUL | NULL | | | userno | varchar(20) | NO | | | | +----------+-------------+------+-----+---------+----------------+ 6 rows in set
添加多列 缺点就是只能放在最后面，不能排在某字段的前面或后面
mysql> alter table user add column(a1 varchar(1),a2 varchar(2)); Database changed Records: 1 Duplicates: 0 Warnings: 0 mysql> describe user; +----------+-------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +----------+-------------+------+-----+---------+----------------+ | age | varchar(2) | YES | | NULL | | | id | smallint(6) | NO | PRI | NULL | auto_increment | | username | varchar(20) | YES | | NULL | | | sex | char(1) | YES | | NULL | | | pid | smallint(6) | YES | MUL | NULL | | | userno | varchar(20) | NO | | | | | a1 | varchar(1) | YES | | NULL | | | a2 | varchar(2) | YES | | NULL | | +----------+-------------+------+-----+---------+----------------+
删除一列
mysql> alter table user drop a1; Database changed Records: 1 Duplicates: 0 Warnings: 0
删除多列用逗号隔开
mysql> alter table user drop sex,drop age; Database changed
修改列
mysql> alter table user modify id int after pid; Database changed Records: 0 Duplicates: 0 Warnings: 0 mysql> show columns from user; +----------+---------------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +----------+---------------------+------+-----+---------+-------+ | username | varchar(20) | YES | | NULL | | | pid | smallint(6) | YES | | NULL | | | id | int(11) | YES | | NULL | | | age | tinyint(3) unsigned | NO | | | | +----------+---------------------+------+-----+---------+-------+ 4 rows in set
修改表名用rename
法一
mysql> alter table user rename deploy; Database changed mysql> show tables; +--------------+ | Tables_in_t1 | +--------------+ | deploy | | dept | | pet | +--------------+ 3 rows in set
用 rename to
mysql> rename table deploy to user; Database changed mysql> show tables; +--------------+ | Tables_in_t1 | +--------------+ | dept | | pet | | user | +--------------+ 3 rows in set
添加主键约束
mysql> create table user(
-> id smallInt,
-> username varchar(20));
Database changed
mysql> alter table user add constraint pk_user_id primary key(id);
Database changed
Records: 0 Duplicates: 0 Warnings: 0
mysql> show create table user;
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------+
| user | CREATE TABLE `user` (
`id` smallint(6) NOT NULL default '0',
`username` varchar(20) default NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------+ 添加唯一约束
mysql> alter table user add unique(username); Database changed Records: 0 Duplicates: 0 Warnings: 0 mysql> describe user; +----------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +----------+-------------+------+-----+---------+-------+ | id | smallint(6) | NO | PRI | 0 | | | username | varchar(20) | YES | UNI | NULL | | +----------+-------------+------+-----+---------+-------+ 2 rows in set添加外键约束
mysql> alter table user add foreign key(pid) references dept(id); Database changed Records: 0 Duplicates: 0 Warnings: 0 mysql> show columns from user; +----------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +----------+-------------+------+-----+---------+-------+ | id | smallint(6) | NO | PRI | 0 | | | username | varchar(20) | YES | UNI | NULL | | | pid | smallint(6) | YES | MUL | NULL | | +----------+-------------+------+-----+---------+-------+ 3 rows in set添加默认约束
mysql> alter table user alter age set default 10; Database changed删除默认约束
mysql> alter table user alter age drop default; Database changed删除主键约束
mysql> alter table user drop primary key; Database changed Records: 0 Duplicates: 0 Warnings: 0删除唯一约束
mysql> show index from user; +-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | +-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ | user | 0 | username | 1 | username | A | 0 | NULL | NULL | YES | BTREE | | | user | 1 | pid | 1 | pid | A | 0 | NULL | NULL | YES | BTREE | | +-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ 2 rows in set mysql> alter table user drop index username; Database changed删除外键约束
mysql> show create table user; +-------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | Table | Create Table | +-------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | user | CREATE TABLE `user` ( `id` smallint(6) NOT NULL default '0', `username` varchar(20) default NULL, `pid` smallint(6) default NULL, `age` tinyint(3) unsigned NOT NULL, KEY `pid` (`pid`), CONSTRAINT `user_ibfk_1` FOREIGN KEY (`pid`) REFERENCES `dept` (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8 | +-------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ 1 row in set mysql> alter table user drop foreign key user_ibfk_1; Database changed Records: 0 Duplicates: 0 Warnings: 0生成外键的时候会有索引，所以这里要把索引也删除掉
mysql> show create table user; +-------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | Table | Create Table | +-------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | user | CREATE TABLE `user` ( `id` smallint(6) NOT NULL default '0', `username` varchar(20) default NULL, `pid` smallint(6) default NULL, `age` tinyint(3) unsigned NOT NULL, KEY `pid` (`pid`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8 | +-------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ 1 row in set mysql> alter table user drop index pid; Database changed Records: 0 Duplicates: 0 Warnings: 0 mysql> show columns from user; +----------+---------------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +----------+---------------------+------+-----+---------+-------+ | id | smallint(6) | NO | | 0 | | | username | varchar(20) | YES | | NULL | | | pid | smallint(6) | YES | | NULL | | | age | tinyint(3) unsigned | NO | | | | +----------+---------------------+------+-----+---------+-------+ 4 rows in set mysql> show create table user; +-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | Table | Create Table | +-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | user | CREATE TABLE `user` ( `id` smallint(6) NOT NULL default '0', `username` varchar(20) default NULL, `pid` smallint(6) default NULL, `age` tinyint(3) unsigned NOT NULL ) ENGINE=InnoDB DEFAULT CHARSET=utf8 | +-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ 1 row in set