Mysql存储过程:如何处理空结果集。

I have written a procedure in which one statement is not executing properly:

我已编写了一个程序，其中有一个语句没有正确执行:

SELECT thumb_image into v_thumb_image FROM RESTAURANT_IMAGE WHERE 
   RESTAURANT_ID = v_restaurant_id

The reason, I investigated is if at any point of time resultset is empty, procedure doesn't run statements further.

我调查的原因是，如果在任何时间点resultset为空，那么过程不会进一步运行语句。

Please note that I am calling this within a LOOP.

请注意，我在一个循环中调用这个。

My concern is not to stop execution if for any v_restaurant_id, resultset is empty.

我关心的是，如果对于任何v_restaurant_id, resultset为空，则不要停止执行。

FULL PROCEDURE:

完整的程序:

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

1 个解决方案

#1

From the documentation:

从文档:

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.

“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，用来控制当游标到达数据集的末尾时会发生什么。如果没有更多的行可用，则不会出现SQLSTATE值'02000'的数据条件。要检测此条件，可以为其设置一个处理程序(或为未找到的条件)。例如，请参阅第13.6.6节“游标”。SELECT…也会出现这种情况。进入var_list语句，不检索行。

So your select from restaurant_image table also meets the NOT FOUND state when it returns no rows, and invokes the defined handler which causes leaving the loop.

因此，如果选择restaurant_image表不返回任何行，并且调用了导致退出循环的已定义处理程序，那么它也会满足未找到的状态。

One solution is to declare another handler for that select by putting it inside a BEGIN...END block:

一个解决方案是声明另一个处理程序，该处理程序将其放入开始…块:

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;

After all, why are doing that using a stored procedure and cursors which would be slow. You can achieve the same functionality executing a single statement:

毕竟，为什么要使用存储过程和游标来实现这一点呢?执行一个语句可以实现相同的功能:

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)

Or make it even faster by eliminating sub queries for every row:

或者通过消除对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

Try these sub-queries separately to find a better understanding.

分别尝试这些子查询以找到更好的理解。

#1