外键约束
概念
特点
定义一个外键时，需要遵守下列规则：
主表必须已经存在于数据库中，或者是当前正在创建的表。
必须为主表定义主键。
主键不能包含空值，但允许在外键中出现空值。也就是说，只要外键的每个非空值出现在指定的主键中，这 个外键的内容就是正确的。
在主表的表名后面指定列名或列名的组合。这个列或列的组合必须是主表的主键或候选键。
外键中列的数目必须和主表的主键中列的数目相同。
外键中列的数据类型必须和主表主键中对应列的数据类型相同。
操作
建立外键约束
create database mydb3; use mydb3; create table if not exists dep ( pid int primary key, name varchar(20) ); create table if not exists per ( id int primary key, name varchar(20), age int, depid int, constraint fok foreign key(depid) references dep(pid) ); create table if not exists dep3 ( pid int primary key, name varchar(20) ); create table if not exists per3 ( id int primary key, name varchar(20), age int, depid int ); alter table per3 add constraint fok3 foreign key(depid) references dep3(pid);
数据插入
必须先给主表添加数据，且从表外键列的值必须依赖于主表的主键列
insert into dep3 values('1001','研发部');
insert into dep3 values('1002','销售部');
insert into dep3 values('1003','财务部');
insert into dep3 values('1004','人事部');
-- 给per3表添加数据
insert into per3 values('1','乔峰',20, '1001');
insert into per3 values('2','段誉',21, '1001');
insert into per3 values('3','虚竹',23, '1001');
insert into per3 values('4','阿紫',18, '1001');
insert into per3 values('5','扫地僧',85, '1002');
insert into per3 values('6','李秋水',33, '1002');
insert into per3 values('7','鸠摩智',50, '1002');
insert into per3 values('8','天山童姥',60, '1003');
insert into per3 values('9','慕容博',58, '1003');
数据删除
主表数据被从表依赖时不能删除，否则可以删除；从表的数据可以随便删除。
如下，第一句和第二句执行成功，第三句执行失败
delete from per3 where depid=1003; delete from dep3 where pid=1004; delete from dep3 where pid=1002;
删除外键约束
语法：alter table 从表drop foreign key 关键词名;
alter table per3 drop foreign key fok3;
多表联合查询
概念
操作
交叉连接查询
select * from dept,emp;
内连接查询
注释；上面是隐式内连接，下面是显式内连接
select * from dept,emp where dept.deptno=emp.dept_id;
select * from dept join emp on dept.deptno=emp.dept_id;
select * from dept join emp on dept.deptno=emp.dept_id and name='研发部';
select * from dept join emp on dept.deptno=emp.dept_id and name='研发部';
select * from dept join emp on dept.deptno=emp.dept_id and (name='研发部' or name='销售部');
select * from dept join emp on dept.deptno=emp.dept_id and (name='研发部' or name ='销售部');
select * from dept join emp on dept.deptno=emp.dept_id and name in ('研发部','销售部');
select a.name,a.deptno,count(*) from dept a join emp on a.deptno=emp.dept_id group by dept_id;
select a.name,a.deptno,count(*) total from dept a join emp on a.deptno=emp.dept_id group by dept_id having total >=3 order by total desc;
外连接查询
若是对应的外表没有数据就补NULL
select * from dept a left join emp b on a.deptno=b.dept_id; select * from dept a right join emp b on a.deptno=b.dept_id; -- select * from dept a full join emp b on a.deptno=b.dept_id; --不能执行 -- 用下面的方法代替上面的full join select * from dept a left join emp b on a.deptno=b.dept_id union select * from dept a right join emp b on a.deptno=b.dept_id; -- 对比union all，发现union all没有去重过滤 select * from dept a left join emp b on a.deptno=b.dept_id union all select * from dept a right join emp b on a.deptno=b.dept_id;
子查询
select * from emp where age<(select avg(age) from emp);
select * from emp a where a.dept_id in (select deptno from dept where name in ('研发部','销售部'));
-- 对比关联查询和子查询如下
select * from emp a join dept b on a.dept_id=b.deptno and (b.name='研发部' and age<30);
select * from (select * from dept where name='研发部') a join (select * from emp where age<30) b on b.dept_id=a.deptno;
子查询关键字
all关键字的用法
select * from emp where age>all(select age from emp where dept_id='1003'); select * from emp a where a.dept_id!=all(select deptno from dept);
any（some）关键字的用法
select * from emp where age>any(select age from emp where dept_id='1003') and dept_id!='1003';
in关键字的用法
select ename,eid from emp where dept_id in (select deptno from dept where name in ('研发部','销售部'));
exists关键字的用法
select * from emp a where a.age<30; select * from emp a where exists(select * from emp where a.age<30); select * from emp a where a.dept_id in (select deptno from dept b); select * from emp a where exists (select * from dept b where a.dept_id = b.deptno);
自关联查询
多表操作总结
到此这篇关于Mysql多表操作方法讲解教程的文章就介绍到这了,更多相关Mysql多表操作内容请搜索脚本之家以前的文章或继续浏览下面的相关文章希望大家以后多多支持脚本之家！