修改数据表

1.修改表名

mysql> show tables;
+----------------+
| Tables_in_test |
+----------------+
| cf             |
| dept01         |
| tb_dept2       |
| tb_emp         |
| tb_emp3        |
| tb_emp6        |
| tb_emp7        |
| tb_emp8        |
| test02         |
+----------------+
9 rows in set (0.00 sec)

mysql> alter table tb_emp8 rename to ladykiller;
Query OK, 0 rows affected (0.05 sec)

mysql> show tables;
+----------------+
| Tables_in_test |
+----------------+
| cf             |
| dept01         |
| ladykiller     |
| tb_dept2       |
| tb_emp         |
| tb_emp3        |
| tb_emp6        |
| tb_emp7        |
| test02         |
+----------------+
9 rows in set (0.00 sec)

2.修改表中数据类型

mysql> show tables;
+----------------+
| Tables_in_test |
+----------------+
| cf             |
| dept01         |
| ladykiller     |
| tb_dept2       |
| tb_emp         |
| tb_emp3        |
| tb_emp6        |
| tb_emp7        |
| test02         |
+----------------+
9 rows in set (0.00 sec)

mysql> desc dept01;
+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| id       | int(11)     | NO   | PRI | NULL    |       |
| name     | varchar(22) | NO   |     | NULL    |       |
| location | varchar(50) | YES  |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

mysql> alter table dept01 modify location int;
Query OK, 0 rows affected (0.25 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc dept01;
+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| id       | int(11)     | NO   | PRI | NULL    |       |
| name     | varchar(22) | NO   |     | NULL    |       |
| location | int(11)     | YES  |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

如果要改变的数据类型转换有误，则会出现

mysql> 
mysql> show tables;
+----------------+
| Tables_in_test |
+----------------+
| cf             |
| dept01         |
| ladykiller     |
| tb_dept2       |
| tb_emp         |
| tb_emp3        |
| tb_emp6        |
| tb_emp7        |
| test02         |
+----------------+
9 rows in set (0.00 sec)

mysql> desc tb_emp
    -> ;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  |     | NULL    |       |
| name  | varchar(25) | YES  |     | NULL    |       |
| tept  | varchar(25) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

mysql> select * from tb_emp;
Empty set (0.00 sec)

mysql> insert into tb_emp values (1,'first','qinghua'),(2,'second','beida'),(3,'thirld',45);
Query OK, 3 rows affected (0.02 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> alter table tb_emp modify tept int;
ERROR 1366 (HY000): Incorrect integer value: 'qinghua' for column 'tept' at row 1

所以说，数据类型需要转换正确

mysql> show tables;
+----------------+
| Tables_in_test |
+----------------+
| cf             |
| dept01         |
| ladykiller     |
| tb_dept2       |
| tb_emp         |
| tb_emp3        |
| tb_emp6        |
| tb_emp7        |
| test02         |
+----------------+
9 rows in set (0.00 sec)

mysql> desc tb_emp
    -> ;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  |     | NULL    |       |
| name  | varchar(25) | YES  |     | NULL    |       |
| tept  | varchar(25) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

mysql> insert into tb_emp values (1,'a',10),(2,'b',20),(3,'c',30);
Query OK, 3 rows affected (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> alter table tb_emp modify tept int;
Query OK, 3 rows affected (0.27 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from tb_emp;
+------+------+------+
| id   | name | tept |
+------+------+------+
|    1 | a    |   10 |
|    2 | b    |   20 |
|    3 | c    |   30 |
+------+------+------+
3 rows in set (0.00 sec)

mysql> 

3.修改字段名

alter table  tb_emp change idd op varchar(20);
语法规则

alter table 表名 change 旧字段名 新字段名 新数据类型

新数据类型不为空 并且遵循数据优先级规则
user-defined data types (highest)
sql_variant
xml
datetimeoffset
datetime2
datetime
smalldatetime
date
time
float
real
decimal
money
smallmoney
bigint
int
smallint
tinyint
bit
ntext
text
image
timestamp
uniqueidentifier
nvarchar (including nvarchar(max) )
nchar
varchar (including varchar(max) )
char
varbinary (including varbinary(max) )
binary (lowest)

4.添加字段

mysql> show tables;
+----------------+
| Tables_in_test |
+----------------+
| cf             |
| dept01         |
| ladykiller     |
| tb_dept2       |
| tb_emp         |
| tb_emp3        |
| tb_emp6        |
| tb_emp7        |
| test02         |
+----------------+
9 rows in set (0.00 sec)

mysql> desc tb_emp;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| op    | varchar(20) | YES  |     | NULL    |       |
| name  | varchar(25) | YES  |     | NULL    |       |
| tept  | int(11)     | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
3 rows in set (0.01 sec)

mysql> alter table tb_emp add sky int first;
Query OK, 0 rows affected (0.17 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> alter table tb_emp add kkk  int not null unique  after name;
ERROR 1062 (23000): Duplicate entry '0' for key 'kkk'
mysql> alter table tb_emp add kkk  int  unique  after name;
Query OK, 0 rows affected (0.24 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> select * from tb_emp;
+------+------+------+------+------+
| sky  | op   | name | kkk  | tept |
+------+------+------+------+------+
| NULL | 1    | a    | NULL |   10 |
| NULL | 2    | b    | NULL |   20 |
| NULL | 3    | c    | NULL |   30 |
+------+------+------+------+------+
3 rows in set (0.00 sec)

5.删除字段

mysql> select * from tb_emp;
+------+------+------+------+------+
| sky  | op   | name | kkk  | tept |
+------+------+------+------+------+
| NULL | 1    | a    | NULL |   10 |
| NULL | 2    | b    | NULL |   20 |
| NULL | 3    | c    | NULL |   30 |
+------+------+------+------+------+
3 rows in set (0.00 sec)

mysql> alter table tb_emp drop sky
    -> ;
Query OK, 0 rows affected (0.23 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> alter table tb_emp drop kkk,tept;
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 'tept' at line 1
mysql> desc tb_emp;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| op    | varchar(20) | YES  |     | NULL    |       |
| name  | varchar(25) | YES  |     | NULL    |       |
| kkk   | int(11)     | YES  | UNI | NULL    |       |
| tept  | int(11)     | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

6.修改字段的排列位置

mysql> desc tb_emp;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| op    | varchar(20) | YES  |     | NULL    |       |
| name  | varchar(25) | YES  |     | NULL    |       |
| kkk   | int(11)     | YES  | UNI | NULL    |       |
| tept  | int(11)     | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

mysql> alter table tb_emp modify name varchar(25) first;
Query OK, 0 rows affected (0.76 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> alter table tb_emp modify  kkk varchar(25) after op;
Query OK, 3 rows affected (0.32 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> desc tb_emp;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| name  | varchar(25) | YES  |     | NULL    |       |
| op    | varchar(20) | YES  |     | NULL    |       |
| kkk   | varchar(25) | YES  | UNI | NULL    |       |
| tept  | int(11)     | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

7.更改表的存储引擎

语法
alter table 表名 ENGINE=引擎

mysql> show create table tb_emp;
+--------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table  | Create Table                                                                                                                                                                                                                   |
+--------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tb_emp | CREATE TABLE `tb_emp` (
  `name` varchar(25) DEFAULT NULL,
  `op` varchar(20) DEFAULT NULL,
  `kkk` varchar(25) DEFAULT NULL,
  `tept` int(11) DEFAULT NULL,
  UNIQUE KEY `kkk` (`kkk`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+--------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> 
mysql> alter table tb_emp engine=MyISAM;
Query OK, 3 rows affected (0.53 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> show create table tb_emp;
+--------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table  | Create Table                                                                                                                                                                                                                   |
+--------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tb_emp | CREATE TABLE `tb_emp` (
  `name` varchar(25) DEFAULT NULL,
  `op` varchar(20) DEFAULT NULL,
  `kkk` varchar(25) DEFAULT NULL,
  `tept` int(11) DEFAULT NULL,
  UNIQUE KEY `kkk` (`kkk`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1 |
+--------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

8.删除外键约束

语法
alter table 表名 drop foreign key 外键名称



mysql> create table tb_emp9
    -> (
    -> id int primary key,
    -> constraint haha foreign key(id) references tb_emp3(id));
Query OK, 0 rows affected (0.10 sec)

mysql> desc tb_emp9;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | NO   | PRI | NULL    |       |
+-------+---------+------+-----+---------+-------+
1 row in set (0.00 sec)

mysql> show create table tb_emp9;
+---------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table   | Create Table                                                                                                                                                                      |
+---------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tb_emp9 | CREATE TABLE `tb_emp9` (
  `id` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  CONSTRAINT `haha` FOREIGN KEY (`id`) REFERENCES `tb_emp3` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+---------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> alter table tb_emp9 drop foreign key haha;
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show create table tb_emp9;
+---------+---------------------------------------------------------------------------------------------------------------+
| Table   | Create Table                                                                                                  |
+---------+---------------------------------------------------------------------------------------------------------------+
| tb_emp9 | CREATE TABLE `tb_emp9` (
  `id` int(11) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+---------+---------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

9.删除表

从表可以直接删除
mysql> create table tb_emp10 ( id int primary key, constraint haha foreign key(id) references tb_emp3(id));;
Query OK, 0 rows affected (0.26 sec)

mysql> drop table tb_emp10;
Query OK, 0 rows affected (0.04 sec)


主表的话，可以先删除从表再删除主表
如果需要保留从表，需要将外键关系解除即可