How to calculate 90th Percentile, SD, Mean for data in SQL - sql

Question

Hi I have a table facility. Which holds a score for each day (Multiple scores can be reported each day and both would be valid)
I need to calculate the 90th percentile, SD, and Mean for score by month.
Facility:
Id Month Date score
1 Jan 1 5
1 Jan 1 5
1 Jan 2 3
1 Jan 3 4
1 Jan 4 4
1 Jan 5 4
1 Feb 1 5
1 Feb 1 5
1 Feb 2 3
1 Feb 3 4
1 Feb 4 4
1 Feb 5 4
Is there any way?
Thanks for your help.

Answer 1

You can use the new suite of analytic functions introduced in SQL Server 2012:
SELECT DISTINCT
[Month],
Mean = AVG(Score) OVER (PARTITION BY [Month]),
StdDev = STDEV(Score) OVER (PARTITION BY [Month]),
P90 = PERCENTILE_CONT(0.9) WITHIN GROUP (ORDER BY Score) OVER (PARTITION BY [Month])
FROM my_table
There are 2 percentile functions: PERCENTILE_CONT for continuous distribution and PERCENTILE_DISC for discrete distribution. Picks one that suits your needs.

Answer 2

Here's the setup...
CREATE TABLE Facility (Id INT NOT NULL, Month nvarchar(3) NOT NULL, Date INT NOT NULL, score INT NOT NULL)
INSERT INTO Facility (Id, Month, Date, score) VALUES (1, 'Jan', 1, 5)
INSERT INTO Facility (Id, Month, Date, score) VALUES (1, 'Jan', 1, 5)
INSERT INTO Facility (Id, Month, Date, score) VALUES (1, 'Jan', 2, 3)
INSERT INTO Facility (Id, Month, Date, score) VALUES (1, 'Jan', 3, 4)
INSERT INTO Facility (Id, Month, Date, score) VALUES (1, 'Jan', 4, 4)
INSERT INTO Facility (Id, Month, Date, score) VALUES (1, 'Jan', 5, 4)
INSERT INTO Facility (Id, Month, Date, score) VALUES (1, 'Feb', 1, 5)
INSERT INTO Facility (Id, Month, Date, score) VALUES (1, 'Feb', 1, 5)
INSERT INTO Facility (Id, Month, Date, score) VALUES (1, 'Feb', 2, 3)
INSERT INTO Facility (Id, Month, Date, score) VALUES (1, 'Feb', 3, 4)
INSERT INTO Facility (Id, Month, Date, score) VALUES (1, 'Feb', 4, 4)
INSERT INTO Facility (Id, Month, Date, score) VALUES (1, 'Feb', 5, 4)
Now, Standard Deviation and Mean are straight forward enough - there are built in aggregate functions for them...
SELECT
[Month],
AVG(CONVERT(real, score)) AS [Mean],
STDEV(score) AS [Standard Deviation]
FROM
Facility
GROUP BY
[Month]
For your 90th percentile, you'll need to invent a function...
CREATE FUNCTION NintythPercentile(#Month nvarchar(3)) RETURNS INT AS
BEGIN
DECLARE #ReturnValue INT
SELECT
#ReturnValue = MIN(DerivedTopTenPercent.score) --AS [90th Percentile]
FROM
(
SELECT TOP 10 PERCENT
score
FROM
Facility
WHERE
[Month] = #Month
ORDER BY
score DESC
) DerivedTopTenPercent
RETURN #ReturnValue
END
With that function in place, your final query will look like this...
SELECT
[Month],
AVG(CONVERT(real, score)) AS [Mean],
STDEV(score) AS [Standard Deviation],
dbo.NintythPercentile([Month]) AS [90th Percentile]
FROM
Facility
GROUP BY
[Month]