SQL 条件 GROUP BY : how to do it?

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

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，按非 NULL 的 billId 分组:

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

我怎样才能做到这一点？顺便说一句，我不关心分组结果的模棱两可的 meetingId。

感谢您的帮助!

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

这比 MIN 效率高一点点如果你真的不关心什么meetingID只要属于正确的组，就会返回。