Mysql在存储过程中嵌套while循环

Question

目前我正在处理一个存储过程，其中我在另一个 while 循环下使用一个 while 循环。但我没有得到预期的结果。最外层的循环是迭代一次。

我正在尝试以下代码。

DELIMITER $$

DROP PROCEDURE IF EXISTS `First_Sp` $$
CREATE DEFINER=`root`@`localhost` PROCEDURE `First_Sp`()
BEGIN

  DECLARE first_while_start  INTEGER DEFAULT 1;
  DECLARE second_while_start INTEGER DEFAULT 1;
  DECLARE first_while_count  INTEGER DEFAULT 3;
  DECLARE second_while_count INTEGER DEFAULT 3;

  WHILE first_while_start < first_while_count
  DO
    WHILE second_while_start < second_while_count
    DO
      SELECT concat(first_while_start,' - ',second_while_start) as result;
      SET second_while_start = second_while_start + 1;
    END WHILE;
    SET first_while_start = first_while_start + 1;
  END WHILE;
END $$

DELIMITER ;  

结果：

mysql> call first_sp();
+--------+
| result |
+--------+
| 1 - 1  |
+--------+
1 row in set (0.00 sec)

+--------+
| result |
+--------+
| 1 - 2  |
+--------+
1 row in set (0.00 sec)

Query OK, 0 rows affected (0.00 sec).

我也尝试过重复循环。但还是没有运气。

DELIMITER $$

DROP PROCEDURE IF EXISTS `First_Sp` $$
CREATE DEFINER=`root`@`localhost` PROCEDURE `First_Sp`()
BEGIN

  DECLARE first_while_start  INTEGER DEFAULT 1;
  DECLARE second_while_start INTEGER DEFAULT 1;
  DECLARE first_while_count  INTEGER DEFAULT 3;
  DECLARE second_while_count INTEGER DEFAULT 3;

  REPEAT
    WHILE second_while_start < second_while_count
    DO
      SELECT concat(first_while_start,' - ',second_while_start) as result;
      SET second_while_start = second_while_start + 1;
    END WHILE;
    SET first_while_start = first_while_start + 1;
  UNTIL first_while_start < first_while_count
  END REPEAT;

END $$

DELIMITER ;

我不是 SQL Developer。我正在努力。

Answer 1

这是因为在外部 while 循环的第一次迭代之后，second_while_start 的值已经是second_while_count，因此内部循环不会执行，因为second_while_start 不再小于second_while_count。

要获得“预期结果”，您需要重置 second_while_start。

DELIMITER $$

DROP PROCEDURE IF EXISTS `First_Sp` $$
CREATE DEFINER=`root`@`localhost` PROCEDURE `First_Sp`()
BEGIN

  DECLARE first_while_start  INTEGER DEFAULT 1;
  DECLARE second_while_start INTEGER DEFAULT 1;
  DECLARE first_while_count  INTEGER DEFAULT 3;
  DECLARE second_while_count INTEGER DEFAULT 3;

  WHILE first_while_start < first_while_count
  DO
    WHILE second_while_start < second_while_count
    DO
      SELECT concat(first_while_start,' - ',second_while_start) as result;
      SET second_while_start = second_while_start + 1;
    END WHILE;
    SET first_while_start = first_while_start + 1;
    /*here comes the important line:*/
    SET second_while_start = 1;
  END WHILE;
END $$

DELIMITER ;