Question

执行以下查询时面临“在查询期间丢失与MySQL服务器的连接”错误。我该如何解决？

SELECT * 
FROM firmalar,
     musterisahipleri,
     notlar 
WHERE firmalar.firmaID NOT IN (
       SELECT m2.firmaID 
       FROM notlar AS n2,
            musterisahipleri AS m2
       WHERE n2.eklemeTarihi > '2013-03-24'
   ) 
   AND musterisahipleri.firmaID = firmalar.firmaID 
   AND notlar.ilgiliID = musterisahipleri.userID;

感谢您的帮助

Answer 1

你正在超时，因为你正在使用效率低下的嵌套子查询。

这样做会更好：

编辑：根据您的上一条评论，此查询将返回自“2013-03-24”以来未添加firmalar条记录的notlar条记录...然后将这些结果加入{{1} 1}}和musterisahipleri再次获取相关的代表和备注（如果适用）

notlar

您还应该确保在您加入的列上有索引，例如

SELECT *
FROM (
    SELECT f.* 
    FROM firmalar AS f
    LEFT JOIN musterisahipleri AS m
        ON m.firmaID = f.firmaID
    LEFT JOIN notlar AS n
        ON n.ilgiliID = m.userID
        AND n.eklemeTarihi > '2013-03-24'
    GROUP BY f.firmaID
    HAVING MAX(n.ilgiliID) IS NULL
) AS f
    LEFT JOIN musterisahipleri AS m
        ON m.firmaID = f.firmaID
    LEFT JOIN notlar AS n
        ON n.ilgiliID = m.userID

Answer 2

您可以通过将not in替换为not exists来加速旧版本的from子查询。适当的指数肯定有帮助（正如Steven Moseley所建议的那样。

此版本将联接移动到not in子句中，并将not exists替换为SELECT * FROM firmalar join musterisahipleri on musterisahipleri.firmaID = firmalar.firmaID join notlar on notlar.ilgiliID = musterisahipleri.userID WHERE not exists (select 1 FROM notlar n2 join musterisahipleri m2 on n2.ilgiliID = m3.userID WHERE n2.eklemeTarihi > '2013-03-24' and firmalar.firmaID = m2.firmaID )：

not in

在编写本文时，我意识到原始查询的问题是join子查询中的表未正确连接在一起。这再次强调了为什么正确的连接语法（使用on子句中的from和where关键字）优于{{1}}子句中的隐式连接。

我该如何修复这个SQL查询？

2 个答案: