database - MySQL IN Clause in SQL query with User Defined Function -

i using following query update children of particular topic.

update topics set reuse = 0 topic_id in (select getchildtopics(187)); 

where

select getchildtopics(187); 

returns "188,190,189" update query updating first row topic_id = 188, instead of updating first topic only, should update 3 topics.

when put values manually works fine.

update topics set reuse = 0 topic_id in (188,190,189); 

can suggest what's wrong doing here?

here code getchildtopics mysql function

    create definer=`root`@`localhost` function `getancestry`(givenid int) returns varchar(1024) charset latin1     deterministic begin     declare rv varchar(1024);     declare cm char(1);     declare ch int;      set rv = '';     set cm = '';     set ch = givenid;     while ch > 0         select ifnull(parent_topic_id,-1) ch         (select parent_topic_id topic_list id = ch) a;         if ch > 0             set rv = concat(rv,cm,ch);             set cm = ',';         end if;     end while;     return rv; end 

try this;)

update topics set reuse = 0 find_in_set(topic_id, getchildtopics(187));