约束条件二

 foreign key,外键

先建被关联的表，并且被关联字段唯一

create table dep(
id int unique not null,
name char(10),
comment char(50)
);

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再创建关联表

create table emp(
id int primary key,
name char(10),
sex enum('male','female'),
dep_id int,
foreign key(dep_id) references dep(id));

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查看表结构

mysql> desc emp;
+--------+-----------------------+------+-----+---------+-------+
| Field  | Type                  | Null | Key | Default | Extra |
+--------+-----------------------+------+-----+---------+-------+
| id     | int(11)               | NO   | PRI | NULL    |       |
| name   | char(10)              | YES  |     | NULL    |       |
| sex    | enum('male','female') | YES  |     | NULL    |       |
| dep_id | int(11)               | YES  | MUL | NULL    |       |
+--------+-----------------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

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先插入被关联表数据，再插入关联表数据

删除先删除关联表数据，再删除被关联表数据。 

建立关联表时添加

on delete cascade #删除同步

on update cascade#更新同步