Question

我有两张桌子：

查找表（tabOne）：

    KEY  |  Group  |  Name  |  Desc  | Val_Key
    ----------------------------------------
     1   |     a   | NameA  | DescA  |  10
     2   |     b   | NameB  | DescB  |  20
     3   |     c   | NameC  | DescC  |  30
     4   |     d   | NameD  | DescD  |  40
     5   |     e   | NameE  | DescE  |  50
     6   |     f   | NameF  | DescF  |  60

包含读数（tabTwo）的第二个表：

    KEY  |  Date  |  Reading  | Val_Key
    ----------------------------------------
     1   |  Date   | Read     |  10
     2   |  Date   | Read     |  20
     3   |  Date   | Read     |  40
     4   |  Date   | Read     |  40
     5   |  Date   | Read     |  30
     6   |  Date   | Read     |  20
     7   |  Date   | Read     |  40
     8   |  Date   | Read     |  20
     9   |  Date   | Read     |  10
     10  |  Date   | Read     |  20
     11  |  Date   | Read     |  50
     12  |  Date   | Read     |  60

我需要做的是与tabTwo一起加入TabOne并创建一个包含最新阅读的列和一个包含TabOne组列中每个项目最早读数的列。

在一天结束时，我想要一张表格如下：

    KEY  |  Group  |  Name  |  Desc  | Val_Key | LastReading | FirstReading |
    -------------------------------------------------------------------------
     1   |     a   | NameA  | DescA  |  10     |             |              |   
     2   |     b   | NameB  | DescB  |  20     |             |              |
     3   |     c   | NameC  | DescC  |  30     |             |              |
     4   |     d   | NameD  | DescD  |  40     |             |              |
     5   |     e   | NameE  | DescE  |  50     |             |              |
     6   |     f   | NameF  | DescF  |  60     |             |              |

谢谢！房地

Answer 1

如果这是Sql Server 2005或更新版本，outer apply将有所帮助：

select TabOne.*, 
       last.Reading LastReading, 
       first.Reading FirstReading
from TabOne
outer apply
(
  select top 1
         Reading
    from TabTwo
   where TabTwo.Val_Key = TabOne.val_Key
  order by TabTwo.Date desc
) last
outer apply
(
  select top 1
         Reading
    from TabTwo
   where TabTwo.Val_Key = TabOne.val_Key
  order by TabTwo.Date asc
) first

Live test is @ Sql Fiddle

Answer 2

如果子查询直接移动到主查询的SELECT子句，那么

@Nikola Markovinović's solution可以更普遍适用，这可能每个子句只检索一个值，因此有效作为标量表达式：

SELECT
  t1.[KEY],
  t1.[Group],
  t1.Name,
  t1.[Desc],
  t1.Val_Key,
  (
    SELECT TOP 1 Reading
    FROM TabTwo
    WHERE Val_Key = t1.Val_Key
    ORDER BY Date DESC
  ) AS LastReading,
  (
    SELECT TOP 1 Reading
    FROM TabTwo
    WHERE Val_Key = t1.Val_Key
    ORDER BY Date ASC
  ) AS FirstReading
FROM TabOne t1

如果您需要，例如沿途的日期，你可能不得不坚持尼古拉的解决方案。有一个替代方案，但它更麻烦（虽然更标准）：它将涉及TabTwo的{{1}}数据分组，以获得每Val_Key个最早/最晚的日期，然后加回Val_Key以访问与找到的日期对应的整行，最后拉出必要的列，最终将两个结果集合并到TabTwo以获得最终的列集。

在SQL Server表中查找最大值和最小值日期值

2 个答案: