SQL Server中的内连接(inner join)

目录

零、码仙励志

一、建库和建表

二、内连接的用法(inner join)

1.select ...... from A,B 的用法

2.select ...... from A,B  where ......的用法(sql92标准)

3.select ...... from A join B on ......的用法(sql99标准)

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零、码仙励志

不被嘲笑的梦想是没有价值的。愿你的梦想不只是空谈的情怀，而是实干的姿态

一、建库和建表

create database scort
use scort
create table emp
(
	empno int primary key,
	ename nvarchar(10),
	sal int,
	deptno int
)
insert into emp values (7369,'smith',3000,20);
insert into emp values (7499,'allen',1500,10);
insert into emp values (7521,'ward',2850,30);
insert into emp values (7566,'jones',2000,30);
insert into emp values (7654,'martin',5000,10);
insert into emp values (7698,'blake',1800,30);

create table dept
(
	deptno int primary key,
	dname nvarchar(10),
	loc nvarchar(10)
)
insert into dept values (10,'accounting','new york');
insert into dept values (20,'research','dallas');
insert into dept values (30,'sales','chicago');
insert into dept values (40,'operations','boston');

create table salgrade
(
	grade int primary key,
	lostl int,
	hisal int
)
insert into salgrade values (1,700,1200);
insert into salgrade values (2,1201,1400);
insert into salgrade values (3,1401,2000);
insert into salgrade values (4,2001,3000);
insert into salgrade values (5,3001,6000);

二、内连接的用法(inner join)

解释：将两个表或两个以上的表以一定的连接条件连接起来，从中检索出满足条件的数据。

注意：inner join 可以简写为 join

1.select ...... from A,B 的用法

1.产生的结果：行数是A和B的乘积，列数是A和B之和。或者说把A表的每一条记录和B表的每一条记录组合在一起，形成的是个笛卡尔积。

2.select ...... from A,B和select ...... from B,A的输出查询结果本质上是一样的，只是看起来A，B表的前后顺序不一样而已。

3.实例演示：

select * from emp,dept

select * from dept,emp

2.select ...... from A,B  where ......的用法(sql92标准)

1.select ...... from A,B  where ......和select ...... fromB,A  where ......查询结果本质上一样

select * from emp,dept where sal>2000

2.如果俩个表或多个表有相同列名的时候，where后面必须加表名。

select * from emp,dept where deptno=10 --error

select * from emp,dept where emp.deptno=10

select * from emp,dept where dept.deptno=10

3.三张表连接在一起

--把工资大于2000的ename，dname和grade输出
select "E".ename,"D".dname,"S".grade
	from emp "E",dept "D",salgrade "S"
	where "E".deptno="D".deptno and
		  ("E".sal>"S".lostl and "E".sal<"S".hisal) and
		  "E".sal>2000

3.select ...... from A join B on ......的用法(sql99标准)

1.select ...... from A join B on ......和select ...... from B join A on ......查询结果本质上一样

2.俩张表连接在一起

select * from emp
	join dept
	on 1=1

select * from emp
	join dept
	on emp.deptno=dept.deptno

3.三张表连接在一起

--把工资大于2000的ename，dname和grade输出
select "E".ename,"D".dname,"S".grade
	from emp "E"
	join dept "D"
	on "E".deptno="D".deptno
	join salgrade "S"
	on "E".sal>"S".lostl and "E".sal<"S".hisal
	where "E".sal>2000

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