Question

I have two tables:

A lookup table (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

A second table containing readings (tabTwo):

    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

What I need to do is join tabTwo with TabOne and create a column with the newest Reading and a column with the oldest reading for each item in the group column of TabOne.

At the end of the day I want a table that look as follow:

    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     |             |              |

Thanks!
Freddie

Answer 1

If this is Sql Server 2005 or newer, outer apply will help:

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.