減去兩個查詢的結果

Question

我有一個查詢數據庫，告訴我每個特定房間總共有多少個按鍵，另一個指示每個特定房間當前登出多少個按鍵。我想創建第三個查詢，指出每個房間現在有多少個可用。

訪問文件：http://jumpshare.com/b/k7pEg4

查詢1（每間鍵）

SELECT r.room_name, count(r.room_id) AS Key_Count 
FROM keys AS k 
INNER JOIN rooms AS r ON r.room_id = k.room_id 
GROUP BY r.room_id, r.room_name 
ORDER BY 2 DESC; 

查詢2（KPR Signedout）

SELECT r.room_name, Count(r.room_id) AS Key_Count 
FROM rooms AS r 
INNER JOIN (keys AS k 
INNER JOIN signin AS s ON k.key_id = s.key_id) ON r.room_id = k.room_id 
WHERE (((s.[return_date]) Is Null)) 
GROUP BY r.room_name, r.room_id 
ORDER BY 2 DESC; 

這是從查詢1和2.如果結果FY218室有8個總鑰匙和2個鑰匙目前已註銷。我可以結合我的查詢來告訴我這個房間還有6把鑰匙嗎？如果不是我怎麼能得到這個結果。

這是我的表和關係：

Answer 1

這應該是你想要什麼：

SELECT r.room_name, Count(k.key_id) AS Key_Count 
FROM rooms AS r 
INNER JOIN keys AS k ON r.room_id = k.room_id 
WHERE k.key_id NOT IN (
    SELECT k2.key_id 
    FROM keys AS k2 
    INNER JOIN signin AS s ON k2.key_id = s.key_id 
    WHERE s.[return_date] Is Null) 
GROUP BY r.room_name 
ORDER BY 2 DESC 

Answer 2

你可以使用子查詢，例如：

SELECT 
r.room_name, 
Count(*) - 
(
    SELECT Count(*) 
    FROM keys AS k2 
    INNER JOIN signin s2 
    ON s2.key_id = k2.key_id 
    WHERE r.room_id = k2.room_id 
) AS AvailableKeys 
FROM rooms AS r 
LEFT JOIN keys AS k 
ON r.room_id = k.room_id 
GROUP BY r.room_name, r.room_id