Mysql多表操作方法講解教程

更新時間：2022年12月03日 09:39:32 作者：coleak

這篇文章主要介紹了MySQL數(shù)據(jù)庫的多表操作，文章圍繞主題展開詳細(xì)的內(nèi)容介紹，具有一定的參考價值，感興趣的小伙伴可以參考一下，希望對你的學(xué)習(xí)有所幫助

外鍵約束

概念

特點

定義一個外鍵時，需要遵守下列規(guī)則：

主表必須已經(jīng)存在于數(shù)據(jù)庫中，或者是當(dāng)前正在創(chuàng)建的表。

必須為主表定義主鍵。

主鍵不能包含空值，但允許在外鍵中出現(xiàn)空值。也就是說，只要外鍵的每個非空值出現(xiàn)在指定的主鍵中，這個外鍵的內(nèi)容就是正確的。

在主表的表名后面指定列名或列名的組合。這個列或列的組合必須是主表的主鍵或候選鍵。

外鍵中列的數(shù)目必須和主表的主鍵中列的數(shù)目相同。

外鍵中列的數(shù)據(jù)類型必須和主表主鍵中對應(yīng)列的數(shù)據(jù)類型相同。

操作

建立外鍵約束

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);

數(shù)據(jù)插入

必須先給主表添加數(shù)據(jù)，且從表外鍵列的值必須依賴于主表的主鍵列

insert into dep3 values('1001','研發(fā)部');
insert into dep3 values('1002','銷售部');
insert into dep3 values('1003','財務(wù)部');
insert into dep3 values('1004','人事部');
-- 給per3表添加數(shù)據(jù)
insert into per3 values('1','喬峰',20, '1001');
insert into per3 values('2','段譽',21, '1001');
insert into per3 values('3','虛竹',23, '1001');
insert into per3 values('4','阿紫',18, '1001');
insert into per3 values('5','掃地僧',85, '1002');
insert into per3 values('6','李秋水',33, '1002');
insert into per3 values('7','鳩摩智',50, '1002'); 
insert into per3 values('8','天山童姥',60, '1003');
insert into per3 values('9','慕容博',58, '1003');

數(shù)據(jù)刪除

主表數(shù)據(jù)被從表依賴時不能刪除，否則可以刪除；從表的數(shù)據(jù)可以隨便刪除。

如下，第一句和第二句執(zhí)行成功，第三句執(zhí)行失敗

delete from per3 where depid=1003;
delete from dep3 where pid=1004;
delete from dep3 where pid=1002;

刪除外鍵約束

語法：alter table 從表drop foreign key 關(guān)鍵詞名;

alter table per3 drop foreign key fok3;

多表聯(lián)合查詢

概念

操作

交叉連接查詢

select * from dept,emp;

內(nèi)連接查詢

注釋；上面是隱式內(nèi)連接，下面是顯式內(nèi)連接

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='研發(fā)部';
select * from dept join emp on dept.deptno=emp.dept_id and name='研發(fā)部';
select * from dept join emp on dept.deptno=emp.dept_id and (name='研發(fā)部' or name='銷售部');
select * from dept join emp on dept.deptno=emp.dept_id and (name='研發(fā)部' or name ='銷售部');
select * from dept join emp on dept.deptno=emp.dept_id and name in ('研發(fā)部','銷售部');
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;

外連接查詢

若是對應(yīng)的外表沒有數(shù)據(jù)就補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; --不能執(zhí)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;
-- 對比union all，發(fā)現(xiàn)union all沒有去重過濾
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;

子查詢

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 ('研發(fā)部','銷售部'));
-- 對比關(guān)聯(lián)查詢和子查詢?nèi)缦?
select * from emp a join dept b on a.dept_id=b.deptno and (b.name='研發(fā)部' and age<30);
select * from (select * from dept where name='研發(fā)部') a join (select * from emp where age<30) b on b.dept_id=a.deptno;

子查詢關(guān)鍵字

all關(guān)鍵字的用法

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）關(guān)鍵字的用法

select * from emp where age>any(select age from emp where dept_id='1003') and dept_id!='1003';

in關(guān)鍵字的用法

select ename,eid from emp where dept_id in (select deptno from dept where name in ('研發(fā)部','銷售部'));

exists關(guān)鍵字的用法

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);

自關(guān)聯(lián)查詢