Problem setting SQL sentence "SELECT... IN " in Query database

Hi there,

I have problem setting up the SQL sentence "SELECT .... IN " in query database smart service. 

if I directly writing "SELECT .... IN ('1234','5678')"  in the SQL Statement, it will work.

if I have create a process variable to be the string '1234','5678', then put the following sentence 

SELECT .... IN (ac!str)       and assign the process variable to this ac!str, it will not work.

I dont know what's the problem here, as I can't hard coded in the SQL sentence, the string need to be dynamic.

Can anyone be able to help?

regards,

Lin