MySQL存储过程，处理多个游标和查询结果

How can I use two cursors in the same routine? If I remove the second cursor declaration and fetch loop everthing works fine. The routine is used for adding a friend in my webapp. It takes the id of the current user and the email of the friend we want to add as a friend, then it checks if the email has a corresponding user id and if no friend relation exists it will create one. Any other routine solution than this one would be great as well.

如何在同一例程中使用两个游标?如果我删除第二个游标声明和获取循环everthing工作正常。该例程用于在我的webapp中添加朋友。它需要当前用户的id和我们想要添加为朋友的朋友的电子邮件,然后检查电子邮件是否具有相应的用户ID,如果不存在朋友关系,则会创建一个。除了这个之外的任何其他常规解决方案也会很棒。

DROP PROCEDURE IF EXISTS addNewFriend;
DELIMITER //
CREATE PROCEDURE addNewFriend(IN inUserId INT UNSIGNED, IN inFriendEmail VARCHAR(80))
BEGIN
    DECLARE tempFriendId INT UNSIGNED DEFAULT 0;
    DECLARE tempId INT UNSIGNED DEFAULT 0;
    DECLARE done INT DEFAULT 0;

    DECLARE cur CURSOR FOR
        SELECT id FROM users WHERE email = inFriendEmail;
    DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = 1;

    OPEN cur;
    REPEAT
        FETCH cur INTO tempFriendId;
    UNTIL done  = 1 END REPEAT;
    CLOSE cur;

    DECLARE cur CURSOR FOR 
        SELECT user_id FROM users_friends WHERE user_id = tempFriendId OR friend_id = tempFriendId;
    DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = 1;

    OPEN cur;
    REPEAT
        FETCH cur INTO tempId;
    UNTIL done  = 1 END REPEAT;
    CLOSE cur;

    IF tempFriendId != 0 AND tempId != 0 THEN
        INSERT INTO users_friends (user_id, friend_id) VALUES(inUserId, tempFriendId);
    END IF;
    SELECT tempFriendId as friendId;
END //
DELIMITER ;

5 个解决方案

#1

Here is a simple example of how to use two cursors in the same routine:

这是一个如何在同一例程中使用两个游标的简单示例:

DELIMITER $$

CREATE PROCEDURE `books_routine`()
BEGIN
  DECLARE rowCountDescription INT DEFAULT 0;
  DECLARE rowCountTitle INT DEFAULT 0;
  DECLARE updateDescription CURSOR FOR
    SELECT id FROM books WHERE description IS NULL OR CHAR_LENGTH(description) < 10;
  DECLARE updateTitle CURSOR FOR
    SELECT id FROM books WHERE title IS NULL OR CHAR_LENGTH(title) <= 10;

  OPEN updateDescription;
  BEGIN
      DECLARE exit_flag INT DEFAULT 0;
      DECLARE book_id INT(10);
      DECLARE CONTINUE HANDLER FOR SQLSTATE '02000' SET exit_flag = 1;

      updateDescriptionLoop: LOOP
        FETCH updateDescription INTO book_id;
            IF exit_flag THEN LEAVE updateDescriptionLoop; 
            END IF;
            UPDATE books SET description = 'Lorem ipsum dolor sit amet, consectetur adipiscing elit.' WHERE books.id = book_id;
        SET rowCountDescription = rowCountDescription + 1;
      END LOOP;
  END;
  CLOSE updateDescription;

  OPEN updateTitle;
  BEGIN
      DECLARE exit_flag INT DEFAULT 0;
      DECLARE book_id INT(10);
      DECLARE CONTINUE HANDLER FOR SQLSTATE '02000' SET exit_flag = 1;

      updateTitleLoop: LOOP
        FETCH updateTitle INTO book_id;
            IF exit_flag THEN LEAVE updateTitleLoop; 
            END IF;
            UPDATE books SET title = 'Lorem ipsum dolor sit amet' WHERE books.id = book_id;
        SET rowCountTitle = rowCountTitle + 1;
      END LOOP;
  END;
  CLOSE updateTitle;

  SELECT 'number of titles updated =', rowCountTitle, 'number of descriptions updated =', rowCountDescription;
END

#2

I know you found a better solution, but I believe the answer to your original question is that you need to SET Done=0; between the two cursors, otherwise the second cursor will only fetch one record before exiting the loop due to Done=1 from the previous handler.

我知道你找到了一个更好的解决方案,但我相信你原来问题的答案是你需要SET Done = 0;在两个游标之间,否则第二个游标只会在退出循环之前获取一个记录,因为前一个处理程序的Done = 1。

#3

I have finally written a different function that does the same thing:

我终于写了一个不同的函数来做同样的事情:

DROP PROCEDURE IF EXISTS addNewFriend;
DELIMITER //
CREATE PROCEDURE addNewFriend(IN inUserId INT UNSIGNED, IN inFriendEmail VARCHAR(80))
BEGIN
 SET @tempFriendId = (SELECT id FROM users WHERE email = inFriendEmail);
 SET @tempUsersFriendsUserId = (SELECT user_id FROM users_friends WHERE user_id = inUserId AND friend_id = @tempFriendId);
 IF @tempFriendId IS NOT NULL AND @tempUsersFriendsUserId IS NULL THEN
  INSERT INTO users_friends (user_id, friend_id) VALUES(inUserId, @tempFriendId);
 END IF;
 SELECT @tempFriendId as friendId;
END //
DELIMITER ;

I hope this is a better solution, it works fine anyway. Thanks for telling me not to use cursors when not necessary.

我希望这是一个更好的解决方案,无论如何都可行。谢谢你告诉我在没有必要的时候不要使用游标。

#4

Rather than using cursors to check for the existence of records, you can use the EXISTS clause in the WHERE clause:

您可以在WHERE子句中使用EXISTS子句,而不是使用游标来检查是否存在记录:

INSERT INTO users_friends 
  (user_id, friend_id) 
VALUES
  (inUserId, tempFriendId)
WHERE EXISTS(SELECT NULL 
               FROM users 
              WHERE email = inFriendEmail)
  AND NOT EXISTS(SELECT NULL 
                   FROM users_friends 
                  WHERE user_id = tempFriendId 
                    AND friend_id = tempFriendId);

I made an alteration after reading Paul's comments about the second query, and reversed the logic so the insert won't add duplicates. Ideally this should be handled as a primary key being a compound key (including two or more columns), which would stop the need for checking in code.

我在阅读了Paul关于第二个查询的评论后做了改动,并颠倒了逻辑,因此插入不会添加重复项。理想情况下,这应作为主键处理,作为复合键(包括两个或更多列),这将停止检入代码的需要。

#5

Wow, i don't know what to say, please go and read about and learn sql a little, no offense but this is amongst the worst SQL i've ever seem.

哇,我不知道该说什么,请去阅读并学习一点sql,没有冒犯,但这是我看似最糟糕的SQL之一。

SQL is a set based language, cursors, in general, are bad, there are situations when they are usefull, but they are fairly rare. Your use of cursors here is totally inappropriate.

SQL是一种基于集合的语言,一般来说,游标很糟糕,有些情况下它们很有用,但它们很少见。你在这里使用游标是完全不合适的。

Your logic in the second cursor is also flawed since it will select any record which inludes the friend, not just the required friendship.

你在第二个光标中的逻辑也是有缺陷的,因为它会选择任何包含朋友的记录,而不仅仅是所需的友谊。

If you wanted to fix it you could try giving the second cursor a differant name, but preferably start over.

如果你想修复它,你可以尝试给第二个光标一个不同的名称,但最好重新开始。

Set a compound PK or unique constraint on users_friends, then you don't have to worry about checking for a relationship, then try something like this.

在users_friends上设置复合PK或唯一约束,然后您不必担心检查关系,然后尝试这样的事情。

INSERT INTO users_friends 
SELECT 
    @inUserId, 
    users.user_id
FROM 
    users
WHERE
    email = @inFriendEmail