SQL 交集 差集 并集 笛卡尔积 应用实例

一、 交集 Join   

1、inner join 其处理结果与等值/自然连接相同  

mssql :      

-- mssql 中 inner join   连接需要 on Connection条件 否则会报错, mysql 不会,其结果等于cross join 
select * from Major m  inner JOIN   
dbo.DepNmOrSubjectNm d on m.DepNoOrSubjectNo =d.DepNoOrSubjectNo 
mysql: 
-- 其结果等于 crose join 
select * from Major m inner join  Department d ; 
-- 等值连接 
select m.*,d.* from Major m inner  join   Department d on m.dptId = d.dptId; 

2、 left join /left outer join 左连接/左外连接    

注：在sql 中, left join 为 left outer join 的缩写    

mssql:

select * from Major m   left  outer  JOIN   
dbo.DepNmOrSubjectNm d on m.DepNoOrSubjectNo =d.DepNoOrSubjectNo 

mysql:

select * from Major m   left    join `department` d on m.dptId = d.dptId; 

3、 right join/right outer join 右连接/右外连接   

mssql:    

select * from Major m   right  outer  JOIN   
dbo.DepNmOrSubjectNm d on m.DepNoOrSubjectNo =d.DepNoOrSubjectNo 

mysql:    

select * from Major m   right outer    join `department` d on m.dptId = d.dptId; 

4、full join 全连接

mssql:

--a 
select * from Major m  FULL OUTER JOIN   
dbo.DepNmOrSubjectNm d on m.DepNoOrSubjectNo =d.DepNoOrSubjectNo 
--b
select * from Major m   left  outer  JOIN   
dbo.DepNmOrSubjectNm d on m.DepNoOrSubjectNo =d.DepNoOrSubjectNo 
union 
select * from Major m   right  outer  JOIN   
dbo.DepNmOrSubjectNm d on m.DepNoOrSubjectNo =d.DepNoOrSubjectNo 
--c 
select * from Major m   left  outer  JOIN   
dbo.DepNmOrSubjectNm d on m.DepNoOrSubjectNo =d.DepNoOrSubjectNo 
union all 
select * from Major m   right  outer  JOIN   
dbo.DepNmOrSubjectNm d on m.DepNoOrSubjectNo =d.DepNoOrSubjectNo 
where m.DepNoOrSubjectNo is null 

mysql:    
注：mysql 5.0版本还不支持 full join ,但可以采用 join + union方法实现,详细可以参考:    
http://www.xaprb.com/blog/2006/05/26/how-to-write-full-outer-join-in-mysql/    

--全连接 == 一个左连接 + 一个右连接 + union(去掉相同行) 
--a
select * from Major m  left outer join `department` d on m.dptId = d.dptId 
union   
select * from Major m  right outer join `department` d on m.dptId = d.dptId 
--b
select * from Major m  left outer join `department` d on m.dptId = d.dptId 
union  all 
select * from Major m  right outer join `department` d on m.dptId = d.dptId 
where m.dptId is null 
--大力提倡使用b，尤其是在处理很大的记录集时, union all 不会进行排序及消除相同的行(消除相同的行可能通过第二个join的条件进行实现),所以可以节省不少时间. 

二、差集 (not in)    

select * from Major m where m.dptId not in(select dptId from department ) 

差集中, mysql 与 mssql 语句可以直接采用 not in 来实现    

三.、并集 union

mssql:    

-- union  并, 默认取消相同 行 
select * from Major m 
union 
select * from Major tm 
-- union full 会有重复记录 
select * from Major m 
union all 
select * from Major tm

mysql:

-- union 默认会取消重复选项 
select * from Major `major` 
union  select * from Major mj ; 
-- union all 不会取消重复选项 
select * from Major m 
union  all 
select * from Major tm 

四、 笛卡尔积    

 mssql:    

-- 笛卡尔积 
select * from Major cross  join   
dbo.DepNmOrSubjectNm 

mysql:    

-- 笛卡尔集 
select * from Major m cross join `department` d; 

带条件的笛卡尔积与等值连接功能相同