Question

我的表

+-----------+---------+
|    Date   |  Letter |
+-----------+---------+
| 13.02.2013|    B    |
| 01.03.2016|    A    |
| 28.12.2003|    C    |
| 12.01.2017|    B    |
| 25.04.2011|    A    |
+-----------+---------+

我创建了一个返回正确数据但不符合预期的查询：

SELECT * from
(
SELECT TOP 1 Date as Date1, Letter as Letter1 from TAB where
Letter = 'A'
order by Date DESC
) TAB
UNION
SELECT * from
(
SELECT TOP 1 Date as Date2, Letter as Letter2 from TAB where 
Letter = 'B'
order by Datum DESC
) TAB

预期产出：

+-----------+---------+-----------+---------+
|    Date1  | Letter1 |    Date2  | Letter2 |
+-----------+---------+-----------+---------+
| 01.03.2016|    A    | 12.01.2017|    B    |
+-----------+---------+-----------+---------+

输出：

+-----------+---------+
|    Date1  | Letter1 |
+-----------+---------+
| 01.03.2016|    A    | 
| 12.01.2017|    B    |
+-----------+---------+

是否可以使用UNION获得所有预期的4行？

谢谢。

Answer 1

如果我理解正确，您似乎想要列中的列表中的＆＃34; A＆＃34;和＆＃34; B＆＃34; s。这实际上不是关系输出，因为一行中的列彼此没有关系。但是，您可以使用条件聚合来完成： TOP 1日期为Date1，Letter为Letter1

select max(case when letter = 'A' then date end) as date1,
       'A' as letter1,
       max(case when letter = 'B' then date end) as date2,
       'B' as letter2
from (select t.*,
             row_number() over (partition by letter order by date desc) as seqnum
      from t
      where letter in ('A', 'B')
     ) t
group by seqnum
order by seqnum;

Answer 2

尝试将第二个查询更改为（作为日期1和字母1）列必须相同

Answer 3

您可以使用join代替union all进行两个子查询，并使用alias

调用两个子查询中的列

Answer 4

如果您需要同一行的结果，则需要连接（而不是联合），例如：

SELECT T1.Date, T1.Letter, T2.date, T2.Letter 
from  (
    SELECT TOP 1 '1' as id,  Date , Letter 
    from TAB 
    where Letter = 'A'
    order by Date DESC
) T1
INNER JOIN ( 
  SELECT TOP 1  '1' as id, Date , Letter  
  from TAB 
  where   Letter = 'B'
  order by Datum DESC
) T2 on T1.id = T2.id

Answer 5

希望我能正确理解你的问题。

请查看以下查询。

select Date1 , Letter1 , Date2 , Letter2
from
(SELECT TOP 1 Date as Date1, Letter as Letter1 from TAB where
Letter = 'A'
order by Date DESC) a join 
(
SELECT TOP 1 Date as Date2, Letter as Letter2 from TAB where 
Letter = 'B'
order by Date DESC
) b;

Answer 6

这不是UNION所做的。它只是将给定的两个数据集合并为一个，如您在示例中所示。您可以尝试使用此答案中的CASE：SQL - Query same column twice with different dates in where clause

使用union后，SQL将列拆分为行

6 个答案: