带有ORDER BY，LIMIT和多个表的UPDATE语法 [英] UPDATE Syntax with ORDER BY, LIMIT and Multiple Tables

问题描述

正在学习SQL，如果这是基本步骤，则对不起.尝试找出适用于以下伪代码的UPDATE解决方案:

Learning SQL, sorry if this is rudimentary. Trying to figure out a working UPDATE solution for the following pseudoish-code:

UPDATE tableA 
SET tableA.col1 = '$var'
WHERE tableA.user_id = tableB.id
AND tableB.username = '$varName'
ORDER BY tableA.datetime DESC LIMIT 1

上面的语法更像SELECT语法，但是基本上是在尝试更新tableA的最新行中的单个列值，该用户名在tableB.username中找到(用$ varName表示) )链接到其tableB.id中的ID号，该ID号作为tableA.user_id中的ID存在.

The above is more like SELECT syntax, but am basically trying to update a single column value in the latest row of tableA, where a username found in tableB.username (represented by $varName) is linked to its ID number in tableB.id, which exists as the id in tableA.user_id.

希望这是有道理的.我猜想某些JOIN是必需的，但是子查询对于UPDATE来说似乎很麻烦.我了解ORDER BY和LIMIT在UPDATE中涉及多个表时是不可用的...但是我需要功能.有办法解决吗?

Hopefully, that makes sense. I'm guessing some kind of JOIN is necessary, but subqueries seem troublesome for UPDATE. I understand ORDER BY and LIMIT are off limits when multiple tables are involved in UPDATE... But I need the functionality. Is there a way around this?

有点困惑，谢谢.

推荐答案

解决方案是将ORDER BY和LIMIT嵌套在FROM子句中，作为连接的一部分.这样一来，您可以先找到要更新的确切行(ta.id)，然后提交更新.

The solution is to nest ORDER BY and LIMIT in a FROM clause as part of a join. This let's you find the exact row to be updated (ta.id) first, then commit the update.

UPDATE tableA AS target
    INNER JOIN (
      SELECT ta.id
      FROM tableA AS ta
        INNER JOIN tableB AS tb ON tb.id = ta.user_id
        WHERE tb.username = '$varName'
        ORDER BY ta.datetime DESC
        LIMIT 1) AS source ON source.id = target.id
    SET col1 = '$var';

给Xaprb的Schwarz男爵的帽子提示有关这个确切主题的出色文章: http://www.xaprb.com/blog/2006/08/10/how-to-use-order-by-and-limit-on-multi-table-updates-in-mysql/

Hat tip to Baron Schwartz, a.k.a. Xaprb, for the excellent post on this exact topic: http://www.xaprb.com/blog/2006/08/10/how-to-use-order-by-and-limit-on-multi-table-updates-in-mysql/

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