create view a_and_b as 
select * from a
union 
select * from b
go

------解决方案--------------------

CREATE  table #A (ID INT,NAME VARCHAR(10),[TIME] SMALLDATETIME)
insert #A  
select 1 ,'zhao', '05/06/2011' UNION ALL
SELECT 2 ,'zhao' ,'05/07/2011'

CREATE table #B (ID INT,NAME VARCHAR(10),[TIME] SMALLDATETIME) 
insert #B  
select 1,'zhao1', '05/05/2011' UNION ALL
SELECT 2,'zhao1', '05/07/2011'

DECLARE @CDATE  SMALLDATETIME
  SET @CDATE='05/05/2011'
  IF EXISTS (SELECT * FROM #A WHERE [TIME]=@CDATE)
	(SELECT * FROM #A WHERE [TIME]=@CDATE)
	ELSE
	(SELECT * FROM #B WHERE [TIME]=@CDATE )
  



ID          NAME       TIME
----------- ---------- -----------------------
1           zhao1      2011-05-05 00:00:00

(1 行受影响)

------解决方案--------------------
SQL code

SELECT * FROM A WHERE TIME = '05/05/2011'
IF @@ROWCOUNT = 0
     SELECT * FROM B WHERE TIEM = '05/05/2011'