SQL Server基础：Join

Posted 2020-06-21

3.Join(麻蛋  废话不多说 有图有真相)

测试数据脚本

 1 CREATE TABLE Atable
 2   ( 
 3      S#    INT, 
 4      Sname nvarchar(32), 
 5      Sage  INT, 
 6      Sfrom  nvarchar(8) 
 7   ) 
 8 
 9 
10  insert into Atable 
11  select 1,N‘刘一‘,18,N‘A‘ union all
12  select 2,N‘钱二‘,19,N‘A‘ union all
13  select 3,N‘张三‘,17,N‘A‘ union all
14  select 4,N‘李四‘,18,N‘A‘ 
15 
16  
17 
18 
19  CREATE TABLE Btable
20   ( 
21      S#    INT, 
22      Sname nvarchar(32), 
23      Sage  INT, 
24      Sfrom  nvarchar(8) 
25   ) 
26 
27 
28  insert into Btable
29  select 1,N‘李四‘,18,N‘B‘ union all
30  select 2,N‘tom‘,19, N‘B‘ union all
31  select 3,N‘刘一‘,17,N‘B‘ union all
32  select 4,N‘jack‘,18,N‘B‘

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数据表

1)cross join：交叉连接不需要任何连接条件。两个表的的数据直接进行笛卡尔积运算。

SELECT * FROM Atable as A  cross join Btable as B order by A.S#

2)inner join == join    内连接的功能是，把两个表相关联的记录列出来，必须是相关联的记录。

SELECT * FROM Atable as A  INNER JOIN Btable as B ON A.Sname = B.Sname

3)left outer join == left join   产生表B的完全集，而A表中匹配的则有值，没有匹配的则以null值取代。

SELECT * FROM Atable as A left outer join Btable as B ON A.Sname = B.Sname

4)right outer join == right jion  产生表A的完全集，而B表中匹配的则有值，没有匹配的则以null值取代。

SELECT * FROM Atable as A right outer join Btable as B ON A.Sname = B.Sname

5)full outer  join  == full join   产生A和B的并集。但是需要注意的是，对于没有匹配的记录，则会以null做为值。

SELECT * FROM Atable as A full join Btable as B ON A.Sname = B.Sname 

6)union与union all    区别就是联合查询的时候union会去重，union all不会去重

SELECT Sname FROM Atable  UNION  SELECT Sname FROM  Btable  
SELECT Sname FROM Atable  UNION ALL SELECT Sname FROM  Btable