此SQL运行不能通过,为什么?

[feburary]

此SQL运行不能通过,为什么?
如果把in换成=则可以通过,为什么?

Declare @temp varchar(100)

select @temp='temp1'

select count(*) from table_name
where (field_name in @temp)

[pengdali]

Declare @temp varchar(100)

select @temp='temp1'

select count(*) from table_name
where field_name in (@temp)

[pengdali]

Declare @temp varchar(100)

select @temp='''temp1'',''temp2'''

exec('select count(*) from table_name where field_name in ('+@temp+')')

[welyngj]

Declare @temp varchar(100)

select @temp='temp1'

exec('select count(*) from table_name
where (field_name in '+@temp)

[gmlxf]

Declare @temp varchar(100),@sql varchar(200)

select @temp='''temp1'',''temp2'''
set @sql='select count(*) from table_name where (field_name in (' + @temp + '))'

exec (@sql)