Question

tblParents

parentId | childId
---------+-----
102      |  1
102      |  3
102      |  4
104      |  3
...

tblPopularity

Id | popularityScore
---+-----
1  | 4000
2  | 8000
3  | 3000
4  | 2000
...

我有一个查询，在给定parentId的情况下找到最受欢迎的childId。我想将几个查询组合在一起以获得一组parentItemId中最受欢迎的项目，例如(102, 104, ...)。

所需的输出

parentId   | mostPopularChildId
           | i.e. the childId with the maximum popularityScore for the given parentId
-----------+-----
102        | 1
104        | 3
...

我怎样才能做到这一点？
它会比运行单个查询快得多吗？

Answer 1

要在SQLite中执行此类查询，您可以计算最高分数，然后联接回表格以获取ID：

select pp.ParentId, po.Id, pp.maxscore
from (select pa.ParentID, max(PopularityScore) as maxscore
      from tblParent pa join
           tblPopularity po
           on pa.ChildId = po.Id
     ) pp join
     tblParent pa
     on pa.ParentId = pp.ParentId join
     tblPopularity po
     on pa.ChildId = po.Id and
        po.PopularityScore = pp.maxscore
where . . .

Answer 2

在SQLite 3.7.11或更高版本中，可以从记录中获取与MIN / MAX匹配的其他列：

SELECT parentId,
       childId AS mostPopularChildId,
       MAX(popularityScore)
FROM tblParents
JOIN tblPopularity ON tblParents.childId = tblPopularity.Id
GROUP BY parentId

如何将几个“MAXIMUM”SQLite查询合并到一个查询中？

2 个答案: