SQL Server 基础：Join用法

Join(麻蛋  废话不多说 有图有真相)

测试数据脚本

 CREATE TABLE Atable
   (
      S#    INT,
      Sname ),
      Sage  INT,
      Sfrom  )
   ) 
 
  insert into Atable
  ,N,N'A' union all
  ,N,N'A' union all
  ,N,N'A' union all
  ,N,N'A' 
 
  CREATE TABLE Btable
   (
      S#    INT,
      Sname ),
      Sage  INT,
      Sfrom  )
   ) 
 
  insert into Btable
  ,N,N'B' union all
  ,N, N'B' union all
  ,N,N'B' union all
  ,N,N'B'

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

通过UNION运算符来将两张表纵向联接，基本方式为：

SELECT 列1 , 列2 FROM 表1
UNION
SELECT 列3 , 列4 FROM 表2;
UNION ALL为保留重复行：
SELECT 列1 , 列2 FROM 表1
UNION ALL
SELECT 列3 , 列4 FROM 表2;

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