Mysql多表操作方法讲解教程

吾爱主题阅读：168 2022-12-06 15:38:00 评论：0

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