剩餘的空間留給項目SQL

Question

我需要一些幫助 問：獲得項目總剩餘空間最好在1個查詢

Grops, items 
Group can contain only MaxAllowed items 

Groups table 
(ID, MAXAllowerdItems) 

Items 
(ID, Group_ID, Name) 

這是不正確的查詢，但起點

select SUM(g.MaxAllowedItems - count(*)), 
from FROM items i, Groups g 
where g.ID=i.Group_ID 
GROUP BY i.Group_ID 
HAVING g.MaxAllowedItems > count(*) 

Answer 1

我認爲你需要這樣的事情：

SELECT 
    groups.ID, 
    MAX(MAXAllowerdItems) - COUNT(items.Group_ID) as remaining_spaces 
FROM 
    groups LEFT JOIN items 
    ON groups.ID = items.Group_ID 
GROUP BY 
    groups.ID 
HAVING 
    remaining_spaces > 0 

MAX(MAXAllowerdItems) will always ha具有相同的MAXAllowerdItems值，並且COUNT(items.Group_ID)將是該組ID的已用行數。

請參閱小提琴here。