Question

Here is my table structure:-

Table01
ID  Name    FkID  dateTime
1   John    1     2012-06-01
2   Peter   2     2012-06-02
3   Penny   2     2012-06-03
4   Patty   3     2012-06-01

I tried this query but 🙁

select * 
from Table01 
where FkID in 
(SELECT FkID FROM Table01 GROUP BY FkID HAVING ( COUNT(PkID) < 2 ))
order by dateTime desc

Desired Results:-

1   John    1     2012-06-01
3   Penny   2     2012-06-03
4   Patty   3     2012-06-01

I need unique results by FkID but from duplicate FkID I need to pick row up by latest datetime. Any guide line please.

Answer 1

You can use ROW_NUMBER to achieve that.

WITH A AS (
  SELECT *, ROW_NUMBER() OVER (PARTITION BY FkID ORDER BY dateTime DESC) AS rn
  FROM Table01 
)
SELECT ID, Name, FkID, dateTime
FROM A
WHERE rn = 1;

I used a Common Table Expression (the WITH statement), but you can do it just as well with a normal subquery. But really, ROW_NUMBER() is a blessing when you are want to work with grouped data (or PARTITION’ed, so to say).