Hi,

        I studied on how joinquery works, used it and found the answer. here's the code for reference of the others:

var detectReq = new GlideRecord('sc_request');

var detectReqItem = detectReq.addJoinQuery('sc_req_item');

detectReq.addQuery('requested_for',user);

detectReqItem.addCondition('cat_item',catalogSysID);

detectReq.query();

        if(detectReq.hasNext()){

                  errorMessage="Previous request detected. You cannot order this request again";

                  result='true';

        }

        else

                  result='false';

Thank you