MySQL 5.7.8+ JSON Array Group By

CREATE TABLE `t1` (
  `jdoc` json DEFAULT NULL
)


insert into t1 values ('[3,4]');
insert into t1 values ('[1,2]');

# https://stackoverflow.com/questions/39906435/convert-json-array-in-mysql-to-rows
SELECT
  JSON_EXTRACT(jdoc, CONCAT('$[', idx, ']')) AS item, count(*)
FROM t1
JOIN ( 
  SELECT  0 AS idx UNION
  SELECT  1 AS idx UNION
  SELECT  2 AS idx UNION
  SELECT  3
  ) AS indexes
WHERE JSON_EXTRACT(jdoc, CONCAT('$[', idx, ']')) IS NOT NULL
GROUP BY 1

# use JSON_TABLE in MySQL 8+