Calculate 90th continuous percentile of each region | SQL

 DROP TABLE IF EXISTS Sales


CREATE TABLE Sales (

 

    Region VARCHAR(50),

    Revenue DECIMAL(10, 2)

);


INSERT INTO Sales ( Region, Revenue) VALUES

 

(  'North', 1000.00),

(  'North', 800.00),

(  'North', 1200.00),

(  'South', 1500.00),

( 'South', 2000.00),

( 'South', 1700.00),

 

( 'East', 1100.00),

(  'East', 900.00),

( 'East', 1300.00),

(  'East', 1150.00),

 

( 'West', 1400.00),

(  'West', 1600.00),

(  'West', 1350.00),

( 'West', 1500.00)

;


--Number of rows (n) = 4

--Desired percentile (P) = 0.9

--Position in the ordered set (k) = (n - 1) * P + 1 = (4 - 1) * 0.9 + 1 = 3.7

--The position k falls between the 3rd and 4th values (1150 and 1300).


--Using linear interpolation:


--Percentile Value = prerow+(k-mod(k))*(currentrow-prerow)

--Percentile Value = 1150+(3.7−3)×(1300−1150)

--Percentile Value=1255

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