mysql的表和约束操作
2018-06-23 13:28:46来源：未知 阅读 ()
在创建表是默认为加上数据引擎和字符集，如创建一个student表，代码如下：
create table students(id int unsigned zerofill auto_increment primary key, name varchar(20) not null, --不允许字段为null sex char(1) );
-----------------------------自动加上数据引擎和字符集------------------------
create table students(id int unsigned zerofill auto_increment primary key,
name varchar(20) not null,
sex char(1)
)engines=innodb default charset=utf8;
- 插入数据(insert和replace)
1.insert命令直接在表的插入一条记录。
insert into students(sname,sex) values('orna','男');
insert into students(sname,sex) values('lisi','男');
insert into students(sname,sex) values('wangwu','男');
mysql> select * from students;
+------------+--------+------+------+
| id | sname | sex | tid |
+------------+--------+------+------+
| 0000000001 | orna | 男 | NULL |
| 0000000002 | lisi | 男 | NULL |
| 0000000003 | wangwu | 男 | NULL |
+------------+--------+------+------+
3 rows in set (0.00 sec)
2.replace命令在插入记录时判断主键是否相同，相同就修改，否则插入一条新记录。
replace into students(id,sname,sex) values(2,'luscy','女'); replace into students(id,sname,sex) values(20,'zhangsan','男'); mysql> select * from students; +------------+----------+------+------+ | id | sname | sex | tid | +------------+----------+------+------+ | 0000000001 | orna | 男 | NULL | | 0000000002 | luscy | 女 | NULL | | 0000000003 | wangwu | 男 | NULL | | 0000000020 | zhangsan | 男 | NULL | +------------+----------+------+------+ 4 rows in set (0.00 sec)
- 用select 结果集创建一个表
mysql> select * from students; +------------+-------+------+ | id | name | sex | +------------+-------+------+ | 0000000001 | luscy | 女 | | 0000000002 | lisi | 男 | | 0000000003 | orna | 男 | +------------+-------+------+ 3 rows in set (0.00 sec) mysql> create table stu select * from students; Query OK, 3 rows affected (0.33 sec) Records: 3 Duplicates: 0 Warnings: 0 mysql> select * from stu; +------------+-------+------+ | id | name | sex | +------------+-------+------+ | 0000000001 | luscy | 女 | | 0000000002 | lisi | 男 | | 0000000003 | orna | 男 | +------------+-------+------+ 3 rows in set (0.00 sec)
- 修改表名
有两种方法可以修改表名。
mysql> rename table stu to st; Query OK, 0 rows affected (0.17 sec) mysql> alter table st rename to s; Query OK, 0 rows affected (0.23 sec)
- 删除表
drop table 表名;
drop table 表1,表2,表3.............. 表示可以同时删除多少个表。
- 修改表的列名称
在修改字段需要注意几点：
- 修改字段宽度只能改大，不能改小；
- 修改字段类型时，要兼容；
mysql> alter table s change name sname varchar(30); Query OK, 3 rows affected (1.02 sec) Records: 3 Duplicates: 0 Warnings: 0 mysql> select * from s; +------------+-------+------+ | id | sname | sex | +------------+-------+------+ | 0000000001 | luscy | 女 | | 0000000002 | lisi | 男 | | 0000000003 | orna | 男 | +------------+-------+------+ 3 rows in set (0.01 sec)
- 限定字段的取值范围
create table students(id int unsigned zerofill auto_increment primary key,
name varchar(20) not null,
sex enum('男','女') --指定取值范围。enum和set关键字同意
);
insert into students(name,sex) values('orna','男'); --插入数据成功
------------------------------------插入数据时报错---------------------------------------
mysql> insert into students(name,sex) values('orna','未知');
ERROR 1265 (01000): Data truncated for column 'sex' at row 1
mysql> select * from students;
+------------+------+------+
| id | name | sex |
+------------+------+------+
| 0000000001 | orna | 男 |
+------------+------+------+
1 row in set (0.00 sec)
- 主建、外建和唯一
一个表只能有一个主键，但可以用多个字段建立一个组合主键。可以有多个外键和唯一约束。
- 增加主键
创建主键的两种方法：
--指定主键名 alter table s add constraint pk primary key(id); --不指定主键名 alter table s add constraint primary key(id);
2.删除主键
mysql> desc s;
+-------+---------------------------+------+-----+------------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------------------+------+-----+------------+-------+
| id | int(10) unsigned zerofill | NO | PRI | 0000000000 | |
| sname | varchar(30) | YES | | NULL | |
| sex | set('男','女') | YES | | NULL | |
+-------+---------------------------+------+-----+------------+-------+
3 rows in set (0.00 sec)
mysql> alter table s drop primary key;
Query OK, 3 rows affected (0.82 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> desc s;
+-------+---------------------------+------+-----+------------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------------------+------+-----+------------+-------+
| id | int(10) unsigned zerofill | NO | | 0000000000 | |
| sname | varchar(30) | YES | | NULL | |
| sex | set('男','女') | YES | | NULL | |
+-------+---------------------------+------+-----+------------+-------+
3 rows in set (0.00 sec)
3.增加字段
在增加字段时，可以用first和after关键字，但没有before关键字。
mysql> alter table students add address varchar(50) not null after sex;
Query OK, 0 rows affected (0.49 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc students;
+---------+---------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+---------+---------------------------+------+-----+---------+----------------+
| id | int(10) unsigned zerofill | NO | PRI | NULL | auto_increment |
| name | varchar(20) | NO | | NULL | |
| sex | set('男','女') | YES | | NULL | |
| address | varchar(50) | NO | | NULL | |
+---------+---------------------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)
4.删除字段
mysql> select * from s; +------------+-------+------+ | id | sname | sex | +------------+-------+------+ | 0000000001 | luscy | 女 | | 0000000002 | lisi | 男 | | 0000000003 | orna | 男 | +------------+-------+------+ 3 rows in set (0.00 sec) mysql> alter table s drop column sex; Query OK, 0 rows affected (0.63 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc s; +-------+---------------------------+------+-----+------------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------------------------+------+-----+------------+-------+ | id | int(10) unsigned zerofill | NO | | 0000000000 | | | sname | varchar(30) | YES | | NULL | | +-------+---------------------------+------+-----+------------+-------+ 2 rows in set (0.00 sec)
2.增加唯一约束
--创建表时增加唯一约束。
create table students(id int unsigned zerofill auto_increment primary key,
sname varchar(20) not null unique,
sex set('男','女'),
tid int unsigned
)engine=innodb default charset=utf8;
--创建表后，再增加唯一约束。
alter table students add constraint uk unique(sname);
mysql> desc students;
+-------+---------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------------------+------+-----+---------+----------------+
| id | int(10) unsigned zerofill | NO | PRI | NULL | auto_increment |
| sname | varchar(20) | NO | UNI | NULL | |
| sex | set('男','女') | YES | | NULL | |
| tid | int(10) unsigned | YES | | NULL | |
+-------+---------------------------+------+-----+---------+----------------+
4 rows in set (0.01 sec)
--删除唯一约束。
mysql> alter table students drop index uk;
Query OK, 0 rows affected (0.26 sec)
Records: 0 Duplicates: 0 Warnings: 0
3.增加外键约束
alter table students add constraint fk foreign key(tid) references teachers(id);
--alter table students add constraint fk foreign key(tid) references teachers(id) on delete set null;表示删除外键引用的记录时把当前记录的外键值修改成空。
--alter table students add constraint fk foreign key(tid) references teachers(id) on delete cascade;表示删除外键引用的记录时把当前记录同时删除。
--alter table students add constraint fk foreign key(tid) references teachers(id) on delete set null on update cascade;表示删除外键引用的记录时把当前记录的外键值修
--改成空，或者修改引用记录同时修改外键引用字段。 mysql> desc students; +-------+---------------------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +-------+---------------------------+------+-----+---------+----------------+ | id | int(10) unsigned zerofill | NO | PRI | NULL | auto_increment | | sname | varchar(20) | NO | | NULL | | | sex | set('男','女') | YES | | NULL | | | tid | int(10) unsigned | YES | MUL | NULL | | +-------+---------------------------+------+-----+---------+----------------+ 4 rows in set (0.00 sec)
4.删除外键约束
alter table students drop foreign key fk; alter table students drop index fk;
5.创建索引
mysql> create index sname on students(sname desc); Query OK, 0 rows affected (0.27 sec) Records: 0 Duplicates: 0 Warnings: 0
--alter table students add index(sname desc);
6.删除索引
alter table students drop index sname;
--drop index sname on students;
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