Postgres 9.4

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CREATE TABLE diary
    (id int, user_id int, custom_foods text)
;
    
INSERT INTO diary
    (id, user_id, custom_foods)
VALUES
    (1, 1, '{"56": 2, "42": 0}'),
    (2, 1, '{"19861": 1}'),
    (3, 2, '{}'),
    (4, 3, '{"331": 0}')
;
4 rows affected

WITH empty AS
  ( SELECT DISTINCT user_id,
                    0 AS COUNT
   FROM diary
   WHERE custom_foods = '{}' )
SELECT user_id,
       count(CASE
                 WHEN VALUE::int > 0 THEN 1
             END)
FROM diary d,
     json_each_text(d.custom_foods::JSON)
GROUP BY user_id
UNION ALL
SELECT *
FROM empty
ORDER BY user_id;


user_id
count


1
2

2
0

3
0