求一SELECT语句！！！

[seahi]

select * from a
left join b on a.xxx=b.xxx
where b.aaa is null or b.aaa<='xxxx'

以上语句中表A和表B中均没有主键，使用该语句会查出多条符合条件的语句。
我希望达到的效果是当b.aaa is null有满足条件的记录时便不再查询出符合条件b.aaa<='xxxx'的记录
只有当b.aaa is null没有满足条件的记录的时候才查询满足条件b.aaa<='xxxx'的记录。

[seahi]

哎,算了,揭帖.

[playwarcraft]

where b.aaa is null
or (b.aaa is not null and b.aaa <='xxxx' )

[wzy_love_sly]

select * from a
left join b on a.xxx=b.xxx
where b.aaa is null

union

select * from a
left join b on a.xxx=b.xxx
where b.aaa is null and b.aaa<='xxxx'

再想想

[areswang]

select * from a
left join b on a.xxx=b.xxx
where b.aaa is null
union all
select * from a
left join b on a.xxx=b.xxx
where b.aaa <='xxxx'

[wzy_love_sly]

select * from a
left join b on a.xxx=b.xxx
where b.aaa is null

union

select * from a
left join b on a.xxx=b.xxx
where b.aaa is not null and b.aaa <='xxxx'

[-狙击手-]

select   *   from   a 
right join   b   on   a.xxx=b.xxx and b.aaa   is   null   
where   b.aaa <='xxxx' 

[areswang]

应该是楼上的。

[wzy_love_sly]

if exists(select 1 from a left join b on a.xxx=b.xxx whereb.aaa is null)
select * from a left join b on a.xxx=b.xxx whereb.aaa is null
else
select * from a left join b on a.xxx=b.xxx where b.aaa <='xxxx'

[seahi]

1楼的查出的结果和我那句没区别，2-4楼更不用说，达不到我的要求，5楼错误，如果bbb不为null则扫描记录也找不出来.
7楼结果虽然对,但由于我的这个语句是嵌套在SELECT语句中的,所以还是不能用,请大家再想想!

[wzy_love_sly]

楼主好猛..................

[seahi]

看来这个问题不好解决,写自定义函数应该会比较容易点吧!?

[seahi]

再问个小儿科问题

ALTER FUNCTION udf_shop_employees_lastshop()
RETURNS @shop_employees_lastshop TABLE
(
emp_id char(10) NOT NULL
)
AS
BEGIN
INSERT @shop_employees_lastshop
select emp_id from shop_employees

RETURN;
END
GO

提示错误信息:
Msg 2010, Level 16, State 1, Procedure udf_shop_employees_lastshop, Line 17
Cannot perform alter on 'udf_shop_employees_lastshop' because it is an incompatible object type.
是何意思啊?自定义函数写的少,没有经验啊.谢啦.