Quartiles in SQL query

Question

I have a very simple table like that:

CREATE TABLE IF NOT EXISTS LuxLog (
  Sensor TINYINT,
  Lux INT,
  PRIMARY KEY(Sensor)
)

It contains

Answer 1

I use this solution with a MYSQL function :

x is the centile you want

array_values your group_concat values order and separated by ,

DROP FUNCTION IF EXISTS centile;

delimiter $$
CREATE FUNCTION `centile`(x Text, array_values TEXT) RETURNS text
BEGIN

Declare DIFF_RANK TEXT;
Declare RANG_FLOOR INT;
Declare COUNT INT;
Declare VALEUR_SUP TEXT;
Declare VALEUR_INF TEXT;

SET COUNT = LENGTH(array_values) - LENGTH(REPLACE(array_values, ',', '')) + 1;
SET RANG_FLOOR = FLOOR(ROUND((x) * (COUNT-1),2));
SET DIFF_RANK = ((x) * (COUNT-1)) - FLOOR(ROUND((x) * (COUNT-1),2));

SET VALEUR_SUP = CAST(SUBSTRING_INDEX(SUBSTRING_INDEX(array_values,',', RANG_FLOOR+2),',',-1) AS DECIMAL);
SET VALEUR_INF = CAST(SUBSTRING_INDEX(SUBSTRING_INDEX(array_values,',', RANG_FLOOR+1),',',-1) AS DECIMAL);

/****
    https://fr.wikipedia.org/wiki/Quantile
    x_j+1 + g (x_j+2 - x_j+1)       
***/
RETURN  Round((VALEUR_INF + (DIFF_RANK* (VALEUR_SUP-VALEUR_INF) ) ),2);

END$$

Example :

Select centile(3/4,GROUP_CONCAT(lux ORDER BY lux SEPARATOR ',')) as quartile_3
FROM LuxLog
WHERE Sensor=12 AND Lux<>0