SQL条件GROUP BY:怎么做？

Question

假设我有以下SQL查询:

SELECT Meeting.id AS meetingId, Bill.id AS billId
FROM Meeting 
LEFT JOIN Bill ON Meeting.FK_BillId = Bill.id

这输出如下:

meetingId | billId
------------------
     a    |  NULL
     b    |  NULL
     c    |     1
     d    |     1
     e    |     1
     f    |     2
     g    |     2

我想要以下输出,billId的组不是NULL:

meetingId | billId
------------------
     a    |  NULL
     b    |  NULL
     c    |     1
     f    |     2

我怎样才能做到这一点？顺便说一下,我并不关心分组结果的模糊会议.

谢谢你的帮助!

Answer 1

在SQL Server:

SELECT  meetingId, billid
FROM    (
        SELECT  ROW_NUMBER() OVER (PARTITION BY billId ORDER BY meetingID) AS rn,
                m.*
        FROM    mytable m
        ) q
WHERE   rn = 1 OR billid IS NULL

ANSI:

SELECT  MIN(meetingId), billid
FROM    mytable
WHERE   billid IS NOT NULL
GROUP BY
        billId
UNION ALL
SELECT  meetingId, billId
FROM    mytable
WHERE   billid IS NULL

MySQL:

SELECT  meetingId, billid
FROM    mytable
WHERE   billid IS NOT NULL
GROUP BY
        billId
UNION ALL
SELECT  meetingId, billId
FROM    mytable
WHERE   billid IS NULL

只要它属于正确的组,MIN如果你真的不关心meetingID将返回什么,这是一种更有效的方法.