sql - MySQL Function Oddness When Using IN Operator -

i have below query runs ok , returns string 2,3. fn_get_plan_upgrade_options returning varchar.

select fn_get_plan_upgrade_options(1); 

now when add function call clause using in operator returns result act_id contains 2 , no results act_id contains 3, there data both both should returned, suggestions or ideas, no errors thrown!?

select act_id , act_name  account_types  act_id in (fn_get_plan_upgrade_options(1)); 

the query want run this:

select ... act_id in (2,3) 

but query running this:

select ... act_id in ('2,3') 

because function returns string (not list of values), , since comparing string number (act_id), mysql automatically cast string number, end running this:

 select ... act_id in (2) 

you can use find_in_set solve problem:

select ... find_in_set(act_id, fn_get_plan_upgrade_options(1))>0