SQLite：連接兩個包含計數表達式的選擇語句

Question

我有一個包含文件夾列表的表，每個文件夾都有一個ID和說明

SELECT * FROM folders

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

我有一個圖像表，其中包含圖像列表，每個圖像都有一個folderid

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

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

我希望能夠編寫一個查詢，該查詢返回所有文件夾的列表及其說明以及其中包含多少個圖像

?????

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

我嘗試了以下查詢：

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，因為它不返回imagecount列。 有任何想法嗎？

Answer 1

這也是一個有效的SQLite查詢，它將返回您的問題的答案：每個文件夾中有多少張圖片？

        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

或用case語句代替sum（），您只可以將count(i.folderid) as imagecount

Answer 2

你近了...

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