datediff的问题
SELECT   *
FROM   table   where   (datediff(ValidPeriodUnit,   CheckTime,   getdate())>   ValidPeriod)

ValidPeriodUnit是nvarchar
ValidPeriod是int
CheckTime是datetime

这个语句就是运行不了,但是把ValidPeriodUnit换成d就可以了，但是数据库里面是动态的,可能是d,yy,mm等,我怎么样用一条sql语句实现以上功能呢，

------解决方案--------------------
declare @ValidPeriodUnit as varchar(10)
set @ValidPeriodUnit = 'day ' -- 'year ', 'month '
declare @sql as varchar(100)
set @sql = 'SELECT * FROM table where (datediff( ' ' ' + @ValidPeriodUnit + ' ' ', CheckTime, getdate())> ValidPeriod)
exec @sql

------解决方案--------------------
动态sql语句基本语法
1 :普通SQL语句可以用Exec执行

eg: Select * from tableName
Exec( 'select * from tableName ')
Exec sp_executesql N 'select * from tableName ' -- 请注意字符串前一定要加N

2:字段名，表名，数据库名之类作为变量时，必须用动态SQL

eg:
declare @fname varchar(20)
set @fname = 'FiledName '
Select @fname from tableName -- 错误,不会提示错误，但结果为固定值FiledName,并非所要。
Exec( 'select ' + @fname + ' from tableName ') -- 请注意 加号前后的 单引号的边上加空格

当然将字符串改成变量的形式也可
declare @fname varchar(20)
set @fname = 'FiledName ' --设置字段名

declare @s varchar(1000)
set @s = 'select ' + @fname + ' from tableName '
Exec(@s) -- 成功
exec sp_executesql @s -- 此句会报错


declare @s Nvarchar(1000) -- 注意此处改为nvarchar(1000)
set @s = 'select ' + @fname + ' from tableName '
Exec(@s) -- 成功
exec sp_executesql @s -- 此句正确

3. 输出参数
declare @num int,
@sqls nvarchar(4000)
set @sqls= 'select count(*) from tableName '
exec(@sqls)
--如何将exec执行结果放入变量中？

declare @num int,
@sqls nvarchar(4000)
set @sqls= 'select @a=count(*) from tableName '
exec sp_executesql @sqls,N '@a int output ',@num output
select @num

------解决方案--------------------
if ValidPeriodUnit = 'day '
SELECT * FROM table where (datediff(day, CheckTime, getdate())> ValidPeriod)
else
if ValidPeriodUnit = 'year '
SELECT * FROM table where (datediff(year, CheckTime, getdate())> ValidPeriod)
else
SELECT * FROM table where (datediff(month, CheckTime, getdate())> ValidPeriod)

------解决方案--------------------
declare @sql VARCHAR(3000)
declare @ValidPeriodUnit varchar(4)
declare @ValidPeriod varchar(100)
create table tab1(id varchar(3),checktime datetime)
select * from tab1
insert tab1
select '001 ',cast( '2007-8-28 ' as datetime)
union all
select '002 ',cast( '2007-8-2 ' as datetime)
select * from tab1
set @ValidPeriodUnit= 'day '
set @ValidPeriod=2
set @sql= '
SELECT *
FROM tab1 where (datediff( '+@ValidPeriodUnit+ ', CheckTime, getdate())> '+@ValidPeriod+ ') '
print @sql
exec(@sql)
drop table tab1



------解决方案--------------------
SELECT *
FROM table
where
(case
when ValidPeriodUnit in ( 'year ', 'yy ', 'yyyy ') then datediff(year, CheckTime, getdate())
when ValidPeriodUnit in ( 'quarter ', 'qq ', 'q ') then dat