Question

我有2个mysql表

表1是“qualitaet-inventar”并且有3000行，其中“sachnummer”为AUTO_INCREMENT

http://s14.directupload.net/images/130421/5qzjmv7b.png

表2是“qualitaet”并且有几行..这里是“id”设置为AUTO_INCREMENT 重要的是“sachnummer”可以存在很多次。（见图中的红色标记）

http://s1.directupload.net/images/130421/yj2qt65d.png

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

我希望通过在“qualitaet”上显示“创建”的最新日期来对所有“sachnummer”进行分组

当“qualitaet”中没有“sachnummer”时，请对“qualitaet-inventar”进行空连接

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

问题是需要很长时间才能得到这个结果

http://s7.directupload.net/images/130421/y32di9hc.jpg

使用此代码可以更快地运行

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

但我没有使用此代码获得孔库存和空白日期

http://s14.directupload.net/images/130421/md6pfsz9.jpg

是否有可能以更快的方式为高表行获取此表？：d

http://s7.directupload.net/images/130421/vzb2hosn.jpg

编辑：

这是我的索引

http://s14.directupload.net/images/130421/gvfgcvp6.jpg

Answer 1

以下不应比现有查询更快地工作 - 但是根据您的描述，可以执行：

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

Answer 2

这是一般的想法，使用较短的表名。

select somefields
from table1 t1 left join table2 t2 on t1.something = t2.something
join (
select something, max(datetimefield) maxdt
from table1
where whatever
group by something
)  t3 on t1.something = t3.something
and t1.datetimefield = maxdt
where whatever

使用“GROUP BY”和MySQL中的最新日期连接多个表

2 个答案: