Question

给出以下结果集：

firstname   lastname    scheduled                 posted          EventAdded
Student      One    2014-04-28 00:00:00.000 2014-04-28 12:02:07.037 NULL
Student      One    2014-04-28 00:00:00.000 2014-04-28 12:02:07.037 later
OtherKid     Two    2014-04-28 00:00:00.000 2014-04-28 21:09:33.457 NULL

如何消除第一条记录？

如果学生有许多EventAddeds - 我们只想要EventAdded不为null的那个。如果学生有许多非空的EventAdded - 我们想要EventAdded不为null的任何记录。如果学生只有一条记录（不管EventAdded是否为空） - 我们想要记录。

我们想要的结果是：

firstname   lastname    scheduled                 posted          EventAdded
Student      One    2014-04-28 00:00:00.000 2014-04-28 12:02:07.037 later
OtherKid     Two    2014-04-28 00:00:00.000 2014-04-28 21:09:33.457 NULL

我们尝试了比我们列出的更多的东西。

Answer 1

如果要对查询执行此操作，可以使用row_number()：

select firstname, lastname, scheduled, posted, eventadded
from (select rs.*,
             row_number() over (partition by firstname, lastname
                                order by (case when eventadded is not null then 1 else 2 end)
                               ) as seqnum
      from ResultSet rs
     ) rs
where seqnum = 1;

Answer 2

如果您有旧服务器（没有row_number（）等）

select T.*  -- replace by delete of course to whack 'em
from tab1 T
where exists (select * from tab1 TIN where 
    (TIN.lastname = T.lastname 
     and TIN.FirstName = T.FirstName and TIN.EventAdded is not null))
and T.EventAdded is null

如何消除不必要的记录

2 个答案: