Mysql存储过程：如何处理空结果集

Question

我编写了一个过程，其中一个语句未正确执行：

SELECT thumb_image into v_thumb_image FROM RESTAURANT_IMAGE WHERE 
   RESTAURANT_ID = v_restaurant_id

我调查的原因是，无论何时结果集为空，过程都不会进一步运行语句。

请注意，我在LOOP中称呼它。

我的担心是如果任何v_restaurant_id结果集为空，也不要停止执行。

完整步骤：

-- --------------------------------------------------------------------------------
-- Routine DDL
-- Note: comments before and after the routine body will not be stored by the server
-- --------------------------------------------------------------------------------
DELIMITER $$

CREATE DEFINER=`root`@`localhost` PROCEDURE `populate_restaurant_details`()
BEGIN
 DECLARE v_finished_cuisines, 
         v_finished, 
         v_restaurant_id, 
         v_count_discount
 INT DEFAULT 0;

 DECLARE v_cuisines, 
         v_thumb_image 
 varchar(200) DEFAULT "";

 DECLARE cuisine_title varchar(50) DEFAULT "";
 -- Fetch all restaurant id
 DECLARE restaurant_cursor CURSOR FOR
   SELECT id FROM delhifoodonline.restaurant order by id desc;

 DECLARE CONTINUE HANDLER 
  FOR NOT FOUND SET v_finished = 1;

 OPEN restaurant_cursor;

 get_restaurant: LOOP   

   FETCH restaurant_cursor INTO v_restaurant_id;
   IF v_finished = 1 THEN 
    LEAVE get_restaurant;
   END IF;

  SET v_finished_cuisines =""; 
  SET v_thumb_image = "";
begin
  DECLARE CONTINUE HANDLER FOR NOT FOUND SET v_thumb_image = NULL;

  SELECT thumb_image into v_thumb_image 
  FROM restaurant_image 
  WHERE restaurant_id = v_restaurant_id
  ORDER BY id
  LIMIT 1;
end;

  SELECT count(*) into v_count_discount FROM restaurant_discount WHERE 
    restaurant_id = v_restaurant_id;

BLOCK2: BEGIN

  DECLARE cuisines_cursor CURSOR FOR 
   SELECT cuisine.title FROM restaurant_cuisine INNER JOIN cuisine 
    ON restaurant_cuisine.cuisine_id = cuisine.id
    WHERE 
    restaurant_cuisine.restaurant_id = v_restaurant_id
    LIMIT 0,5;

  DECLARE CONTINUE HANDLER 
    FOR NOT FOUND SET v_finished_cuisines = 1;
  SET v_cuisines = "";
  OPEN cuisines_cursor;

  get_cuisine: LOOP
   FETCH cuisines_cursor INTO cuisine_title;

   IF v_finished_cuisines = 1 THEN 
    LEAVE get_cuisine;
   END IF;

   SET v_cuisines = CONCAT(cuisine_title,", ",v_cuisines);

   END LOOP get_cuisine;
  CLOSE cuisines_cursor;

END BLOCK2;

  SET v_cuisines = TRIM(BOTH ", " FROM v_cuisines);

  IF v_count_discount > 0 THEN
   SET v_count_discount = 1;
  ELSE
   SET v_count_discount = 0;
  END IF;

  UPDATE restaurant SET 
                        thumb_image = v_thumb_image,
                        cuisines_list = v_cuisines,
                        discount_available = v_count_discount
                   WHERE id= v_restaurant_id;
 END LOOP get_restaurant;

CLOSE restaurant_cursor;

END

Answer 1

从文档中：

NOT FOUND是以'02'开头的SQLSTATE值类的简写。这在游标的上下文中是相关的，用于控制游标到达数据集末尾时发生的情况。如果没有更多行可用，则发生SQL值为'02000'的“无数据”条件。要检测这种情况，您可以为其设置处理程序（或为NOT FOUND条件）。有关示例，请参见第13.6.6节“游标”。对于不检索行的SELECT ... INTO var_list语句，也会发生这种情况。

因此，从restaurant_image表中进行选择NOT FOUND时，如果它不返回任何行，那么它也会满足状态，并调用已定义的处理程序，从而导致退出循环。

一种解决方案是通过将选择放入一个BEGIN...END块中来声明该选择的另一个处理程序：

begin
  DECLARE CONTINUE HANDLER FOR NOT FOUND SET v_thumb_image = NULL;

  SELECT thumb_image into v_thumb_image 
  FROM restaurant_image 
  WHERE restaurant_id = v_restaurant_id
  ORDER BY id
  LIMIT 1;
end;

毕竟，为什么要使用存储过程和游标来做到这一点呢？您可以通过执行一条语句来实现相同的功能：

UPDATE restaurant 
SET thumb_image = (
    SELECT thumb_image 
    FROM restaurant_image 
    WHERE restaurant_id = restaurant.id
    ORDER BY id
    LIMIT 1),
discount_available = IF(EXISTS(
    SELECT 1
    FROM restaurant_discount 
    WHERE restaurant_id = restaurant.id), 1, 0), 
cuisines_list = (
    SELECT group_concat(cuisine.title separator ', ')
    FROM restaurant_cuisine
    INNER JOIN cuisine ON restaurant_cuisine.cuisine_id = cuisine.id
    WHERE restaurant_cuisine.restaurant_id = restaurant.id
    LIMIT 0,5)

或者通过消除每一行的子查询来使其更快：

UPDATE restaurant r
LEFT JOIN 
    (SELECT restaurant_id, count(*) AS discount_available
    FROM restaurant_discount 
    GROUP BY restaurant_id) d ON r.id = d.restaurant_id
LEFT JOIN 
    (SELECT restaurant_id, thumb_image 
    FROM restaurant_image r1
    WHERE NOT EXISTS (
        SELECT 1 FROM restaurant_image r2 WHERE r2.restaurant_id = r1.restaurant_id AND r2.id < r1.id
    )) t ON r.id = t.restaurant_id
LEFT JOIN
    (SELECT rc.restaurant_id, SUBSTRING_INDEX(GROUP_CONCAT(c.title SEPARATOR ', '), ',', 5) AS cuisines_list
    FROM restaurant_cuisine rc
    INNER JOIN cuisine c ON rc.cuisine_id = c.id
    GROUP BY rc.restaurant_id
    ) rc ON r.id = rc.restaurant_id
SET r.discount_available = IF(d.discount_available = 0, 0, 1),
r.thumb_image = t.thumb_image,
r.cuisines_list = rc.cuisines_list

分别尝试这些子查询以获取更好的理解。