Question

I suppose this is not so hard but I can not get it.

For example I have table T1:

ID    
-----
1000
1001

And I have table T2:

ID      GROUP  DATE
--------------------------
1000    ADSL   2.2.2012
1000    null   3.2.2012
1000    NOC    4.2.2012
1001    NOC    5.2.2012
1001    null   6.2.2012
1001    TV    7.2.2012

I want to select from T1 only the row that has as GROUP value NOC from T2 but only if NOC group is for the minimum DATE value in T2.

So my result in this case would be only 1001 because for its minimum DATE 5.2.2012 Group is NOC!

I do not want any joins and I can not use default values for IDs (where id=1000 or id=1001) because this is just example of some big table.

Important also is that I can not use t1.id = t2.id because in some application where I am using this I can not write the whole SQL expression but only partial. I can only use id.

I tried something like:

select id 
from t1 
where
   id in (select id from t2 
          where group = 'NOC' 
          and date in (select min(date) from t2 
                       where id in (select id from t1) 
                      )
         )

But this does not work.

I know it seems little confusing but I really can’t use where t1.id = t2.id

Thanks

Answer 1

If T2.ID is a foreign key referencing T1.ID, you don’t really need the T1 table, because all the IDs could be obtained from T2 only:

SELECT o.ID
FROM T2 AS o
WHERE EXISTS (
  SELECT MIN(i.DATE)
  FROM T2 AS i
  WHERE i.ID = o.ID
  HAVING MIN(i.DATE) = o.DATE
)
WHERE o."GROUP" = 'NOC'

But if you insist on involving T1, you just need to modify the above like this:

SELECT *
FROM T1
WHERE ID IN (
  SELECT o.ID
  FROM T2 AS o
  WHERE o."GROUP" = 'NOC'
    AND EXISTS (
      SELECT MIN(i.DATE)
      FROM T2 AS i
      WHERE i.ID = o.ID
      HAVING MIN(i.DATE) = o.DATE
    )
)