How can I calculate the median of values in SQLite?

Question

I\'d like to calculate the median value in a numeric row. How can I do that in SQLite 4?

Answer 1

Dixtroy provided the best solution via group_concat(). Here is a full sample for this:

DROP TABLE [t];
CREATE TABLE [t] (name, value INT);
INSERT INTO t VALUES ('A', 2);
INSERT INTO t VALUES ('A', 3);
INSERT INTO t VALUES ('B', 4);
INSERT INTO t VALUES ('B', 5);
INSERT INTO t VALUES ('B', 6);
INSERT INTO t VALUES ('C', 7);

results into this table:

name|value
A|2
A|3
B|4
B|5
B|6
C|7

now we use the (slightly modified) query from Dextroy:

SELECT name, --string_list, count, middle,
    CASE WHEN count%2=0 THEN
        0.5 * substr(string_list, middle-10, 10) + 0.5 * substr(string_list, middle, 10)
    ELSE
        1.0 * substr(string_list, middle, 10)
    END AS median
FROM (
    SELECT name, 
        group_concat(value_string,"") AS string_list,
        count() AS count, 
        1 + 10*(count()/2) AS middle
    FROM (
        SELECT name, 
            printf( '%010d',value) AS value_string
        FROM [t]
        ORDER BY name,value_string
    )
    GROUP BY name
);

...and get this result:

name|median
A|2.5
B|5.0
C|7.0