Calculating percentile rankings in MS SQL

Question

What's the best way to calculate percentile rankings (e.g. the 90th percentile or the median score) in MSSQL 2005?

I'd like to be able to select the 25th, median, and 75th percentiles for a single column of scores (preferably in a single record so I can combine with average, max, and min). So for example, table output of the results might be:

Group  MinScore  MaxScore  AvgScore  pct25  median  pct75 -----  --------  --------  --------  -----  ------  ----- T1     52        96        74        68     76      84 T2     48        98        74        68     75      85 

Answer 1

I would think that this would be the simplest solution:

SELECT TOP N PERCENT FROM TheTable ORDER BY TheScore DESC 

Where N = (100 - desired percentile). So if you wanted all rows in the 90th percentile, you'd select the top 10%.

I'm not sure what you mean by "preferably in a single record". Do you mean calculate which percentile a given score for a single record would fall into? e.g. do you want to be able to make statements like "your score is 83, which puts you in the 91st percentile." ?

EDIT: OK, I thought some more about your question and came up with this interpretation. Are you asking how to calculate the cutoff score for a particular percentile? e.g. something like this: to be in the 90th percentile you must have a score greater than 78.

If so, this query works. I dislike sub-queries though, so depending on what it was for, I'd probably try to find a more elegant solution. It does, however, return a single record with a single score.

-- Find the minimum score for all scores in the 90th percentile SELECT Min(subq.TheScore) FROM (SELECT TOP 10 PERCENT TheScore FROM TheTable ORDER BY TheScore DESC) AS subq 

Answer 2

Check out the NTILE command -- it will give you percentiles pretty easily!

SELECT  SalesOrderID,      OrderQty,     RowNum = Row_Number() OVER(Order By OrderQty),     Rnk = RANK() OVER(ORDER BY OrderQty),     DenseRnk = DENSE_RANK() OVER(ORDER BY OrderQty),     NTile4  = NTILE(4) OVER(ORDER BY OrderQty) FROM Sales.SalesOrderDetail  WHERE SalesOrderID IN (43689, 63181)