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Calculating PERCENTILE in SQL Server

How to find 25th percentile or 75th percentile of given set of numbers (records in table) in SQL Server 2005?

In this post I will show couple of ways of calculating Percentile. I am not a statistician so i just took the help of YouTube to learn on how Percentile is actually calculated. During that course I found two different formulas for calculating Percentile! So I have provided couple of methods to calculate Percentile based on both the formulas I learnt.


IF  EXISTS (SELECT * FROM sys.objects 
WHERE object_id = OBJECT_ID(N'[dbo].[tblPercentile]') AND type in (N'U'))
DROP TABLE [dbo].[tblPercentile]
GO


CREATE TABLE tblPercentile
(
Sno INT identity(1,1), iValue float
)
GO

--First set of sample records
Insert into tblPercentile
SELECT 2 UNION ALL SELECT 2 UNION ALL
SELECT 3 UNION ALL SELECT 4 UNION ALL
SELECT 5 UNION ALL SELECT 5 UNION ALL
SELECT 5 UNION ALL SELECT 6 UNION ALL
SELECT 7 UNION ALL SELECT 8 UNION ALL
SELECT 8 UNION ALL SELECT 8 UNION ALL
SELECT 8 UNION ALL SELECT 8 UNION ALL
SELECT 9 UNION ALL SELECT 9 UNION ALL
SELECT 10 UNION ALL SELECT 11 UNION ALL
SELECT 11 UNION ALL SELECT 12

--Second dataset for testing
--Insert into tblPercentile
-- SELECT 22 UNION ALL SELECT 22 UNION ALL
-- SELECT 23 UNION ALL SELECT 24 UNION ALL
-- SELECT 24 UNION ALL SELECT 24 UNION ALL
-- SELECT 25 UNION ALL SELECT 27 UNION ALL
-- SELECT 27 UNION ALL SELECT 28 UNION ALL
-- SELECT 30 UNION ALL SELECT 33
GO

--Solution:
/*
Method 1:
Reference URL - http://www.youtube.com/watch?v=Snf6Wpn-L4c&NR=1
*/


--Variable declaration
DECLARE @percentile int
DECLARE @tmp float

--Assign the PERCENTILE value which you want to calculate
Set @percentile = 25


SELECT 
@tmp = @percentile/100.0 * (COUNT(*)+1) 
FROM tblPercentile


SELECT 
CASE WHEN COUNT(ivalue) > 1 THEN SUM(ivalue)/2.0 
    ELSE MAX(iValue) 
END AS [Percentile] 
FROM tblPercentile
WHERE Sno BETWEEN FLOOR(@tmp) AND CEILING(@tmp)

/* 
Method 2:
Reference URL - http://www.thefreemathtutor.com/thepercentileformula.html
*/

SELECT iValue AS [Percentile] FROM tblPercentile
WHERE sno = (
SELECT CEILING(@percentile/100.0 * (COUNT(*))) 
FROM tblPercentile
)
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