Mysql 过程中的游标不返回任何值

我有一个创建临时表的数据库，这些表名随机生成并保存在checkout.unid中。我想删除所有这些表并截断表签出。我认为最漂亮的解决方案是一个程序，但它不会起作用：

DROP PROCEDURE IF EXISTS `spCheckoutsCleanup`;
CREATE PROCEDURE `spCheckoutsCleanup` ()
  SQL SECURITY INVOKER
BEGIN
    DECLARE `t` VARCHAR(64);
    DECLARE `ch` CURSOR FOR SELECT `unid` FROM `Checkouts`;
    OPEN `ch`;

    drop_tables: LOOP
        FETCH `ch` INTO `t`;
        DROP TABLE IF EXISTS `t`;
    END LOOP;

    CLOSE `ch`;
    TRUNCATE TABLE `Checkouts`;
END

---

我总是不获取数据-获取、选择或处理的行数为零，尽管这些表在那里，而且表签出也不是空的。

为了结束循环，您必须添加以下内容：

DECLARE CONTINUE HANDLER FOR NOT FOUND SET ...;

请参见中的示例。例如

DROP PROCEDURE IF EXISTS `spCheckoutsCleanup`;
CREATE PROCEDURE `spCheckoutsCleanup` ()
  SQL SECURITY INVOKER
BEGIN
    DECLARE done INT DEFAULT FALSE;
    DECLARE `t` VARCHAR(64);
    DECLARE `ch` CURSOR FOR SELECT `unid` FROM `Checkouts`;
    DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;

    OPEN `ch`;

    drop_tables: LOOP
        FETCH `ch` INTO `t`;
        IF done THEN
           LEAVE drop_tables;
        END IF;
        DROP TABLE IF EXISTS `t`;
    END LOOP;

    CLOSE `ch`;
    TRUNCATE TABLE `Checkouts`;
END

---

否则，一旦到达光标的末端，就会出现错误。

我用以下方法处理它：

DROP PROCEDURE IF EXISTS `spCheckoutsCleanup`;
CREATE PROCEDURE `spCheckoutsCleanup` ()
  SQL SECURITY INVOKER
BEGIN
    DECLARE done int DEFAULT 0;
    DECLARE t CHAR(64);
    DECLARE ch CURSOR FOR SELECT `unid` FROM `Checkouts`;
    DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = 1;
    OPEN ch;

    drop_table: LOOP
        FETCH ch INTO t;
        IF done = 1 THEN
            LEAVE drop_table;
        END IF;
        SET @sql := CONCAT('DROP TABLE IF EXISTS `', t, '`'); 
        PREPARE dropt FROM @sql; 
        EXECUTE dropt;
    END LOOP;

    CLOSE ch;
    TRUNCATE TABLE `Checkouts`;
END;
CALL spCheckoutsCleanup;