Question

我有一个包含文件夹列表的表，每个文件夹都有一个ID和描述

SELECT * FROM folders

_id |   description
---------------------------
1   |   default
2   |   test

我有一个包含图像列表的图像表，每个图像都有一个文件夹

select folderid, count(*) as imagecount from images GROUP BY folderid;

folderid|   imagecount
---------------------------
1       |   4
2       |   5

我希望能够编写一个查询，返回所有文件夹的列表及其描述，以及其中有多少图像

?????

_id |   description |   imagecount
------------------------------------------------------
1   |   default     |   4
2   |   test        |   5

我尝试了以下查询：

SELECT _id, description from folders, (select folderid, count(*) as imagecount from images GROUP BY folder) WHERE _id = folderid;

但是它不起作用，因为它不返回imagecount列。有什么想法吗？

Answer 1

这也是一个有效的SQLite查询，它将返回您的问题的答案：每个文件夹中有多少图像？

        select f.description, sum ( case when i.folderid is null then 0 else 1 end ) as imagecount 
        from folders f left join images i on i.folderid = f.id
        group by f.description

或者使用case语句代替sum（），你可以count(i.folderid) as imagecount

Answer 2

你很亲密......

SELECT _id, description, imagecount
from folders,
(select folderid, count(*) as imagecount from images GROUP BY folderid)
WHERE _id = folderid;

SQLite：连接两个包含计数表达式的select语句

2 个答案: