如何在 MySQL 存储过程中使用准备好的语句？

Question

我正在使用 mysql，我需要以某种方式使用后面查询中准备好的语句返回的列 curid。我使用准备好的语句，因为正如我所读到的，它是将变量传递给 LIMIT 子句的唯一方法。我在这里有这个存储过程：

DROP PROCEDURE IF EXISTS fixbalance;
CREATE PROCEDURE fixbalance (userid INT)
  BEGIN
  DECLARE i INT DEFAULT 0;
  DECLARE balance INT DEFAULT 0;
  DECLARE idcnt INT;

  SET idcnt = (SELECT COALESCE(COUNT(id), 0) 
               FROM coupon_operations 
               WHERE user_id = userid);
  IF idcnt <> 0 THEN
    WHILE i <= idcnt DO
      BEGIN
        SET @iter = i;
        SET @user_id = userid; 
        SET @sql = CONCAT('SELECT id AS curid 
                           FROM coupon_operations 
                           WHERE user_id = ? 
                           ORDER BY id ASC 
                           LIMIT ?, 1');
        PREPARE stmt FROM @sql;
        EXECUTE stmt USING @user_id, @iter;
        DEALLOCATE PREPARE stmt;
        SET balance = balance + (SELECT points 
                                 FROM coupon_operations 
                                 WHERE user_id = @user_id 
                                 AND id = @curid);
        UPDATE coupon_operations SET balance = balance;
        SET i = i + 1;
      END;
    END WHILE;
  END IF;
END;
|

这不起作用-我不确定如何传递curid。

Answer 1

解决方案是在准备好的语句本身中设置变量，如下所示：

SET @sql = CONCAT('SET @curid = SELECT id
                                FROM coupon_operations 
                                WHERE user_id = ? 
                                ORDER BY id ASC 
                                LIMIT ?, 1');

Answer 2

我很高兴你找到了答案。另一种解决方案是使用SELECT...INTO语法：

SET @sql = CONCAT('SELECT id INTO @curid FROM coupon_operations 
                   WHERE user_id = ? 
                   ORDER BY id ASC 
                   LIMIT ?, 1');