Select rows based on two columns in SQL Server - sql

Question

I have a table which stores data where accidentally data has been stored multiple times because of case sensivity for the username field on server side code. The username field should be regarded as case insensitive. The important columns and data for the table can be found below.
My requirements now is to delete all but the most recent saved data. I'm writing an sql script for this, and started out by identifying all rows that are duplicates. This selection returns a table like below.
For each row, the most recent save is LASTUPDATEDDATE if it exist, otherwise CREATEDDATE. For this example, the most recent save for 'username' would be row 3.
ID CREATEDDATE LASTUPDATEDDATE USERNAME
-- ----------- --------------- --------
1 11-NOV-11 USERNAME
2 01-NOV-11 02-NOV-11 username
3 8-JAN-12 USERname
My script (which selects all rows where a duplicated username appears) looks like:
SELECT
id, createddate, lastupdateddate, username
FROM
table
WHERE
LOWER(username)
IN
(
SELECT
LOWER(username)
FROM
table
GROUP BY
LOWER(username)
HAVING
COUNT(*) > 1
)
ORDER BY
LOWER(username)
My question now is: How do I select everything but row 3? I have searched Stack Overflow for a good match to this question, but found no match good enough. I know I probably have to make a join of some kind, but can't really get my head around it. Would be really thankful for a push in the right direction.
We are using SQL Server, probably a quite new version.

Answer 1

To delete duplicates, you can use:
with todelete as (
select t.*,
row_number() over (partition by lower(username) order by createddate desc) as seqnum
from table
)
delete from t
where seqnum > 1
This assigns a sequential number to each row, starting with 1 for the most recent. It then deletes all but the most recent.
For two dates, you can use:
with todelete as (
select t.*,
row_number() over (partition by lower(username) order by thedate desc) as seqnum
from (select t.*,
(case when createddate >= coalesdce(updateddate, createddate)
then createddate
else updateddate
end) as thedate
from table
) t
)
delete from t
where seqnum > 1

Answer 2

A couple of things to note -- there is no reason to use LOWER in your query. A = a in SQL Server.
Also, to get the correct date, you can use COALESCE to determine if LastUpdatedDate exists and if so, sort by it, else sort by CreatedDate.
Putting that together, this should work:
DELETE T
FROM YourTable T
JOIN (
SELECT *, ROW_NUMBER() OVER (PARTITION BY username
ORDER BY COALESCE(lastupdateddate, createddate) DESC) as RN
FROM YourTable
) T2 ON T.Id = T2.Id
WHERE T2.RN > 1
Here is a sample fiddle: http://www.sqlfiddle.com/#!3/51f7c/1
As #Gordon correctly suggests, you could also use a CTE depending on the version of SQL Server you use (2005+):
WITH CTE AS (
SELECT *, ROW_NUMBER() OVER (PARTITION BY username
ORDER BY COALESCE(lastupdateddate, createddate) DESC) as RN
FROM YourTable
)
DELETE FROM CTE WHERE RN > 1