MySQL从SELECT语句返回值

我经常会遇到这样的错误: 结果包含多行

DROP FUNCTION IF EXISTS db.GetUserIDByCourseID;
CREATE FUNCTION db.`GetUserIDByCourseID`(CourseID int) RETURNS int(11)
BEGIN
      SELECT (c.user_id + COALESCE(pi.user_id, 0) + COALESCE(p.user_id, 0))
        INTO @user_id
        FROM courses c
             LEFT JOIN users u
                ON u.user_id = c.user_id
             LEFT JOIN partners p
                ON p.partner_id = c.partner_id
             LEFT JOIN partners_individual pi
                ON pi.individual_id = c.individual_id;
             WHERE c.course_id = CourseID;

      SELECT user_type_id
        INTO @user_type_id
        FROM users
       WHERE user_id = @user_id;

      RETURN @user_type_id;
   END;