update 问题
在表A中有两列
A1,   A2
01       a01
02       a01
01       b01
02       b02
...
表B中也有两列  
B1,   B2
a01     b01
b01     c01
b02     c02
....
现在想把表B中与A表的A2与B1相同的数据用B2替换
就是想把A表换成
A1,   A2
01       b01
02       b01
01       c01
02       c02


------解决方案--------------------
drop table A,B
go
create table A(A1 varchar(10),A2 varchar(10))
insert into A
select '01 ', 'a01 '
union all select '02 ', 'a01 '
union all select '01 ', 'b01 '
union all select '02 ', 'b02 '
create table B(B1 varchar(10),B2 varchar(10))
insert into B
select 'a01 ', 'b01 '
union all select 'b01 ', 'c01 '
union all select 'b02 ', 'c02 '

update A
set A2=B.B2
from A
inner join B on A.A2=B.B1

select * from A
/*
A1 A2
---------- ----------
01 b01
02 b01
01 c01
02 c02

（所影响的行数为 4 行）

*/
------解决方案--------------------

declare @bb table (A1 varchar(20),A2 varchar(20))
insert into @bb
select '01 ', 'a01 '
union all select '02 ', 'a01 '
union all select '01 ', 'b01 '
union all select '02 ', 'b02 '
declare @cc table(B1 varchar(20),B2 varchar(20))
insert into @cc
select 'a01 ', 'b01 '
union all select 'b01 ', 'c01 '
union all select 'b02 ', 'c02 '
select bb.A1 as A1,cc.B2 as A2
from @bb bb
left join @cc cc on bb.A2=cc.B1