SQL语句有关问题，如何查结果都一样_Sql Server

SQL语句问题，怎么查结果都一样
select *
from DQ_Rp_Plan_Houseitem
where datediff(dd,cast('2012-04-12' as datetime),cast('2012-04-15' as datetime)) between 1 and 5

和

select *
from DQ_Rp_Plan_Houseitem
where datediff(dd,cast('2012-04-12' as datetime),cast('2012-04-19' as datetime)) between 1 and 5

和

select *
from DQ_Rp_Plan_Houseitem
where datediff(dd,cast('2012-04-12' as datetime),cast('2012-04-15' as datetime)) between 1 and 10

怎么查结果都一样结果都是数据表里的全部数据没有按指定时间显示

哪里的问题？

数据库的日期字段为字符串型的 vchar

------解决方案--------------------

SQL code

select *from DQ_Rp_Plan_Houseitemwhere datediff(dd,cast('2012-04-12' as datetime),cast('2012-04-15' as datetime)) between 1 and 5--datediff(dd,cast('2012-04-12' as datetime),cast('2012-04-15' as datetime))=3,3在1和5之间，条件为真，但会所有结果和select *from DQ_Rp_Plan_Houseitemwhere datediff(dd,cast('2012-04-12' as datetime),cast('2012-04-19' as datetime)) between 1 and 5 --datediff(dd,cast('2012-04-12' as datetime),cast('2012-04-19' as datetime))=7，不在1和5之间，返回空集和select *from DQ_Rp_Plan_Houseitemwhere datediff(dd,cast('2012-04-12' as datetime),cast('2012-04-15' as datetime)) between 1 and 10--datediff(dd,cast('2012-04-12' as datetime),cast('2012-04-15' as datetime))=3，条件成立，返回所有结果
------解决方案--------------------
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如果用
SQL code



select *
from DQ_Rp_Plan_Houseitem
where datediff(dd,cast('2012-04-12' as datetime),getdate()) between 1 and 5




不提示错误 但会没有结果

------解决方案--------------------
SQL codeselect *from DQ_Rp_Plan_Houseitemwhere datediff(dd,cast('2012-04-12' as datetime),getdate()) between 1 and 5--datediff(dd,cast('2012-04-12' as datetime),getdate())值为17--不在1到5的范围内，条件不成立，所以表里面的内容查不出来
------解决方案--------------------
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select *
from DQ_Rp_Plan_Houseitem
where datediff(dd,cast('2012-04-12' as datetime),cast('2012-04-15' as datetime)) between 1 and 5

和


select *
from DQ_Rp_Plan_Houseitem
where datediff(dd,c……