How to use SELECT Query in MYSQL TRIGGER

DROP TRIGGER IF EXISTS `updateInstancesResults`;
DELIMITER //
CREATE TRIGGER `updateInstancesResults` AFTER INSERT ON `form_results`
 FOR EACH ROW BEGIN
    DECLARE showPendingInstance INT(10);
    UPDATE instances SET response_status = 'taken' WHERE instance_id = NEW.instance_id;
    SET showPendingInstance     = (SELECT `value` FROM `attribute_xref` WHERE form_id = NEW.form_id AND `attribute_id` =2 LIMIT 1);
    IF (showPendingInstance = 0) THEN
           UPDATE instances SET response_status = 'skipped', status = 0 WHERE form_id = NEW.form_id
        AND response_status = 'pending'
        AND instance_id < NEW.instance_id;
    END IF;
  END
//
DELIMITER ;