加入基於在多個表中的條件

Question

三個表，列如下：

A: A_id, B_id, C_id, flag, ... 
B: B_id, date, ... 
C: C_id, date 

如果A.flag爲NULL，則我需要從A中的所有行加入了與B關於B_ID具有B.date在過去。

如果A.flag不是NULL，那麼我需要A的所有行與B_id上的B在過去具有C.date，C在C_id上連接。

嘗試：

SELECT * 
FROM A, B, C 
WHERE A.A_id = B.B_id 
AND ((A.flag IS NULL AND (NOW() > B.date) OR 
    (A.flag IS NOT NULL AND (NOW() > C.date) AND C.C_id = A.C_id)) 

但我需要在A.flag is NULL行一些條件來阻止它與C.每行這是我不能工作了加盟。

還是有更簡單的方法來做到這一點？

Answer 1

你可以嘗試

SELECT a.*, b.* 
FROM a INNER JOIN b ON a.B_id = b.B_id 
WHERE a.flag IS NULL AND b.date < NOW() 
UNION 
SELECT a.*, b.* 
FROM a INNER JOIN b ON a.B_id = b.B_id 
INNER JOIN c ON a.C_id = c.C_id 
WHERE a.flag IS NOT NULL AND c.date < NOW() 

Answer 2

你這可能會做的伎倆！

SELECT a.* 
FROM A AS a 
LEFT JOIN B AS b ON b.b_id = a.b_id AND NOW() > b.date AND a.flag IS NULL 
LEFT JOIN C AS c ON c.c_id = a.c_id AND NOW() > c.date AND a.flag IS NOT NULL 

如果你只是想從A符合以下任一條件的行，你需要添加一個地方是這樣的：

WHERE b.b_id IS NOT NULL OR c.c_id IS NOT NULL 

否則，你將最終在A中的所有行:)

完整的查詢會是這樣：

SELECT a.* 
FROM A AS a 
LEFT JOIN B AS b ON b.b_id = a.b_id AND NOW() > b.date AND a.flag IS NULL 
LEFT JOIN C AS c ON c.c_id = a.c_id AND NOW() > c.date AND a.flag IS NOT NULL 
WHERE b.b_id IS NOT NULL OR c.c_id IS NOT NULL 

Answer 3

像這樣的東西可能會奏效，以及：

SELECT * 
FROM A 

WHERE A.flag IS NULL 
    AND EXISTS (SELECT * FROM B WHERE date < NOW() AND B_id = A.A_id) 

    OR A.flag IS NOT NULL 
    AND EXISTS (SELECT * FROM C WHERE date < NOW() AND C_id = A.A_id)