使用"GROUP BY"将多个表连接起来和MySQL中的最新日期 [英] join multiple tables with "GROUP BY" and latest date in MySQL

问题描述

我有2个mysql表

表1是"qualitaet-inventar"，具有3000行，其中"sachnummer"为AUTO_INCREMENT

table 1 is "qualitaet-inventar" and has 3000 rows with "sachnummer" as AUTO_INCREMENT

表2是"qualitaet"，并且有几行..这里的"id"设置为AUTO_INCREMENT 重要的是"sachnummer"可以存在很多次. (请参见图片中的红色标记)

table 2 is "qualitaet" and has a couple of rows.. here is "id" set to AUTO_INCREMENT the important thing is that "sachnummer" can be exist many times. (see the red marks in picture)

我想做的是从"qualitaet"到"qualitaet-inventar"的左连接

what i want to do is a left join from "qualitaet" to "qualitaet-inventar"

我想通过在"qualitaet"上显示创建"的最新日期来对所有"sachnummer"进行分组

i want to group all "sachnummer" by showing the latest date of "created" on "qualitaet"

并且当"qualitaet"中没有"sachnummer"时，请空连接"qualitaet-inventar"

and when a "sachnummer" isn't available in "qualitaet" do an empty join to "qualitaet-inventar"

所以我有这个代码

SELECT 
    i.`sachnummer` AS id,
    MAX(q.`created`) AS letztemessung,
    i.`sachnummer-name` AS sachnummer
FROM
        `qualitaet-inventar` i
LEFT JOIN
        `qualitaet` q on i.`sachnummer` = q.`sachnummer`
GROUP BY
        sachnummer
ORDER BY
    sachnummer ASC

问题是要花很长时间才能得到这个结果

the problem is that it needs a long time to get this result

使用此代码，它的运行速度更快

with this code it works faster

SELECT 
    q.`sachnummer` AS id,
    MAX(q.`created`) AS letztemessung,
    i.`sachnummer-name` AS sachnummer
FROM
    qualitaet q
LEFT JOIN
    `qualitaet-inventar` i on q.`sachnummer` = i.`sachnummer`
GROUP BY
    sachnummer
ORDER BY
    sachnummer ASC

但是我没有得到此代码的孔库存和空日期

but i don't get the hole inventory and an empty date with this code

是否有可能以更快的方式获得较高的表行数? :D

is it possible to get this table in a faster way for high table-rows? :D

这是我的索引

推荐答案

以下不应的运行速度比现有查询快-但从您的描述来看，可能做:

The following shouldn't work any faster than your existing query - but from your description, may do:

SELECT i.`sachnummer` AS id,
       MAX(q.`created`) AS letztemessung,
       i.`sachnummer-name` AS sachnummer
FROM `qualitaet-inventar` i
JOIN `qualitaet` q on i.`sachnummer` = q.`sachnummer`
GROUP BY sachnummer
UNION ALL
SELECT i.`sachnummer` AS id,
       q.`created` AS letztemessung,
       i.`sachnummer-name` AS sachnummer
FROM `qualitaet-inventar` i
LEFT JOIN `qualitaet` q on i.`sachnummer` = q.`sachnummer`
WHERE q.`sachnummer` IS NULL
ORDER BY sachnummer ASC

另一种可能性:

select id, max(letztemessung) as letztemessung, max(sachnummer) as sachnummer
from
(SELECT `sachnummer` AS id,
        `created` AS letztemessung,
        '' AS sachnummer
 FROM `qualitaet` q
 UNION ALL
 SELECT `sachnummer` AS id,
        cast(null as datetime) AS letztemessung,
        `sachnummer-name` AS sachnummer
 FROM `qualitaet-inventar` i
) sq
group by id 
ORDER BY sachnummer ASC

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