Mysql：用什么代替 WITH 语句或 TEMPORARY 表？

Question

有没有办法将选择查询存储到表中？我通常使用 WITH 语句 但这仅适用于 mysql 8+。我目前正在使用 mysql 5.1。因此，我尝试实现一个临时表，但不能在同一个查询中多次引用一个临时表（如下面的查询）。

CREATE TEMPORARY TABLE test
SELECT  TIMESTAMPDIFF(MONTH, MAX(L.period), CURDATE()) as timediff, L.contractID 
FROM license L GROUP BY L.contractID;

UPDATE `contract` AS C
SET C.valid = CASE
WHEN (SELECT timediff FROM test AS T WHERE T.contractID = C.contractID) < 1 THEN 2
WHEN (SELECT timediff FROM test AS T WHERE T.contractID = C.contractID) = 1 THEN 1
WHEN (SELECT timediff FROM test AS T WHERE T.contractID = C.contractID) > 1 THEN 0
END;

DROP TEMPORARY TABLE test;

Answer 1

您可以通过将更新表述为联接来立即解决您的问题：

UPDATE contract c
INNER JOIN test t
    ON t.contractID = c.contractID
SET valid = CASE WHEN t.timediff < 1 THEN 2
                 WHEN t.timediff = 1 THEN 1
                 ELSE 0 END;

这种方法解决了多次引用临时表的问题。