Mysql存储过程: how to handle empty result set

Question

我写了一个过程，其中一条语句没有正确执行:

SELECT thumb_image into v_thumb_image FROM RESTAURANT_IMAGE WHERE 
   RESTAURANT_ID = v_restaurant_id

我调查的原因是，如果在任何时间点结果集为空，程序就不会进一步运行语句。

请注意，我是在循环中调用它。

我担心的是，如果对于任何 v_restaurant_id，结果集为空，则不要停止执行。

完整程序:

-- --------------------------------------------------------------------------------
-- 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

来自documentation :

NOT FOUND is shorthand for the class of SQLSTATE values that begin with '02'. This is relevant within the context of cursors and is used to control what happens when a cursor reaches the end of a data set. If no more rows are available, a No Data condition occurs with SQLSTATE value '02000'. To detect this condition, you can set up a handler for it (or for a NOT FOUND condition). For an example, see Section 13.6.6, “Cursors”. This condition also occurs for SELECT ... INTO var_list statements that retrieve no rows.

因此，当您从 restaurant_image 表中选择的内容没有返回任何行时，它也满足 NOT FOUND 状态，并调用导致退出循环的已定义处理程序。

一个解决方案是通过将它放在 BEGIN...END block 中为该选择声明另一个处理程序:

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，为什么要使用会很慢的存储过程和游标来做到这一点。您可以通过执行单个语句实现相同的功能:

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更好地理解。