How To Retrieve The Top 50% Of A Column - or, any other %-age.

Question

How do, folks.
If the title is somewhat too vague, lemme 'splain...
Basically, let's say I have a data set where one of the columns is an amount paid - Amt_Pd
- and another column is the date when that transaction took place -
Date_Pd - and another identifying the member who paid the amount -
Mbr_ID.
I then set up a query to select all rows where the Date_Pd meets a particular criteria; say,
WHERE YEAR(Date_Pd) = 2014, summing the Amt_Pd
values for each Mbr_ID; Tot_Pd. Then, I order the result with ORDER BY Tot_Pd DESC.
Now, what I'm trying to attempt is to be able to define a certain percentage - to remain true to the title, let's set that to 50% - and then only return records up until the 50% mark for
Tot_Pd is reached.
Can this be done in SQL?

Answer 1

I think this is what you're looking for:
DECLARE @table TABLE (amountPaid FLOAT, datePaid DATE, memberID INT)
INSERT INTO @table (amountPaid, datePaid, memberID) VALUES
(10.00, '2014-01-01',1),(10.00, '2014-02-01',2),(10.00, '2014-03-01',3),(10.00, '2014-04-01',4),(10.00, '2014-05-01',5),(10.00, '2014-06-01',6),(10.00, '2014-07-01',7),(10.00, '2014-08-01',8),(10.00, '2014-01-01',9),(10.00, '2014-10-01',10)
SELECT *
FROM (
SELECT *, SUM(amountPaid) OVER (ORDER BY datePaid ROWS UNBOUNDED PRECEDING) AS runningTotal, ((SELECT SUM(amountPaid) FROM @table) / 100) * 50 AS percentileAmount
FROM @table
) x
WHERE percentileAmount >= runningTotal
It depends upon SQL Server 2012's windowed functions. It would be possible to also do this with a rCTE in earlier versions.
Giving your code an eyball, I think I see what it's doing. Unfortunately, I'm stuck using 2008 R2, so I don't have access to UNBOUNDED or PRECEDING. I'll have to research rCTE - I know what a CTE is, but not the 'r' prefix.