MySQL 5.7

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

CREATE TABLE features (id INT, val JSON)
SELECT 1 id, '["apple", "banana", "orange"]' val
UNION ALL SELECT 2, '["apple", "orange", "pineapple"]'
UNION ALL SELECT 3, '["orange", "banana"]'
UNION ALL SELECT 4, '[]';
SELECT *
FROM features;


id
val


1
["apple", "banana", "orange"]

2
["apple", "orange", "pineapple"]

3
["orange", "banana"]

4
[]


SELECT DISTINCT JSON_EXTRACT(features.val, CONCAT('$[', numbers.num, ']')) arr_values
FROM features, ( SELECT 0 num UNION ALL
                 SELECT 1 UNION ALL
                 SELECT 2 UNION ALL
                 SELECT 3 UNION ALL
                 SELECT 4 UNION ALL
                 SELECT 5 UNION ALL
                 SELECT 6 UNION ALL
                 SELECT 7 UNION ALL
                 SELECT 8 UNION ALL
                 SELECT 9 ) numbers
HAVING arr_values IS NOT NULL;


arr_values


"apple"

"orange"

"banana"

"pineapple"


CREATE PROCEDURE get_unique ()
BEGIN
CREATE TEMPORARY TABLE temp (val JSON);
INSERT INTO temp 
    SELECT val 
    FROM features;
CREATE TEMPORARY TABLE tmp (val JSON);
cycle: LOOP
    INSERT IGNORE INTO tmp
        SELECT JSON_EXTRACT(val, '$[0]')
        FROM temp;
    DELETE 
        FROM temp
        WHERE JSON_EXTRACT(val, '$[1]') IS NULL;
    UPDATE temp
        SET val = JSON_REMOVE(val, '$[0]');
    IF 0 = (SELECT COUNT(*)
                FROM temp) THEN
        LEAVE cycle;
    END IF;
END LOOP;
DROP TEMPORARY TABLE temp;
SELECT DISTINCT * 
    FROM tmp
    WHERE val IS NOT NULL; 
DROP TEMPORARY TABLE tmp;
END
CALL get_unique();


val


"apple"

"orange"

"banana"

"pineapple"

id	val
1	["apple", "banana", "orange"]
2	["apple", "orange", "pineapple"]
3	["orange", "banana"]
4	[]