这样 为什么提示shrq无效？解决思路

这样 为什么提示shrq无效？
SELECT a.Jydbh,  
  a.mddd,CONVERT(varchar(12) , a.Jhrq, 112 ) as shrq,
  a.Jhrq,
b.Clbh 

  FROM Jydxx as a left outer join Fhxx as b on a.Jydbh=b.Jydbh and 
  left(shrq,8) >= '20120403' and left(shrq,8) <= '20120408'

这样 为什么提示shrq无效

[解决办法]

SQL code

SELECT     a.Jydbh,       a.mddd,CONVERT(varchar(12) , a.Jhrq, 112 ) as shrq,    a.Jhrq,    b.ClbhFROM Jydxx as a left outer join Fhxx as b on a.Jydbh=b.Jydbh and  --left(shrq,8) >= '20120403' and left(shrq,8) <= '20120408'datediff(day, '2012-04-03', a.shrq)>=0 and datediff(day, '2012-04-08', a.shrq)<=0