Question

三个表，列如下：

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

如果A.flag为NULL，那么我需要来自A的所有行与B_id上的B一起加入过去的B.date。

如果A.flag不是NULL，那么我需要A中的所有行与B_id上的B连接，过去有C.date，C在C_id上连接。

尝试：

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))

但是我需要A.flag is NULL行中的某些条件来阻止它加入来自C的每一行。这是我无法解决的问题。

或者有更简单的方法吗？

Answer 1

你可以尝试

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

这可能会为你做到这一点！

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

如果您只希望A中符合这些条件的行，则需要添加如下所示的位置：

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

否则你最终会得到A中的所有行。：）

完整的查询将是：

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

这样的事情也可能有效：

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)

根据多个表中的条件连接

3 个答案: