如何避免错误子查询返回sqlyog中的多行

Question

这里我想返回结果中的所有行，但是在执行查询后它会给出错误。

Error Code : 1242    Subquery returns more than 1 row

提前谢谢你......！

BEGIN

    DECLARE TODAYS DATE;
    DECLARE RESULTDAYS DATE;
    DECLARE ISSUEDAYS DATE;
    DECLARE DAYS INT;

    SET TODAYS=CURDATE();
    SET ISSUEDAYS= (SELECT issue_date FROM bookstore.issue_book WHERE 
    return_date IS NULL AND status = 0);

    SET DAYS=(TODAYS-ISSUEDAYS);    
    SELECT DAYS;

   END$$

DELIMITER ;