----------------------------
-- Author  :fredrickhu(小F，向高手学习)
-- Date    :2011-04-06 09:47:40
-- Verstion:
--      Microsoft SQL Server 2008 (RTM) - 10.0.1600.22 (Intel X86) 
--    Jul  9 2008 14:43:34 
--    Copyright (c) 1988-2008 Microsoft Corporation
--    Enterprise Edition on Windows NT 5.1 <X86> (Build 2600: Service Pack 3)
--
----------------------------
--> 测试数据：[a]
if object_id('[a]') is not null drop table [a]
go 
create table [a]([a] int,[b] int)
insert [a]
select 1,4 union all
select 2,5 union all
select 3,6
--> 测试数据：[b]
if object_id('[b]') is not null drop table [b]
go 
create table [b]([a] int,[b] int)
insert [b]
select 2,5 union all
select 3,6 union all
select 4,7
--------------开始查询--------------------------

select * from [a] full join b on a.a=b.a


select * from a
union all
select * from b
----------------结果----------------------------
/* 
a    b    a    b
1    4    NULL    NULL
2    5    2    5
3    6    3    6
NULL    NULL    4    7

a    b
1    4
2    5
3    6
2    5
3    6
4    7
*/

------解决方案--------------------
探讨

union:行集合并
full join:列合并,两张表的数据会全部出来(不管满不满足条件)


------解决方案--------------------
union会重复的值删除
full join不删除重复值


------解决方案--------------------
union all 不会干掉重复记录，只有union才会干掉。

------解决方案--------------------
探讨

当然结果不同了
union会把table本身重复的也干掉
full join不会


------解决方案--------------------
探讨

最通俗的说法就~一个为合并列~一个行集合并