String variable inside "IN" keyword of SQL

If you want to get all the records corresponding to the ID's, we can use a simple sql query with IN keyword.

Select * from Table_1 WHERE ID IN (1001,1002,1005,1006)

This query is simple as it looks. But when i want to replace the numbers with a string,
declare @pm varchar(100)
set @pm='1001,1002'
SELECT * FROM table2 WHERE groupid IN(@pm)

then it will raise an error "Conversion failed when converting the varchar value '1001,1002' to data type int."

To avoid this problem, we can change the all sql query to a string and execute it.
ie,

declare @pm varchar(100)
set @pm='1001,1002'

declare @pm1 varchar(100)
set @pm1='SELECT * FROM table2 WHERE groupid IN('
set @pm=@pm1 + @pm + ')'

exec(@pm)
I think this is the simple way!!!
Waiting for feedback.....