Retrieving nested values from PostgreSQL JSON columns - json

Question

Our database contains a table "log" like this:
id | purchases (json)
1 {"apple":{"price":5,"seller":"frank"}, "bred":{"price":3,"seller":"kathy"}}
2 {"milk":{"price":3,"seller":"anne"}, "banana":{"price":2,"seller":"frank"}}
3 {"bred":{"price":4,"seller":"kathy"}}
We would like to retrieve all records containing "seller":"kathy". We tried simple queries like this:
SELECT id FROM log WHERE purchases ->> 'seller' LIKE 'kathy'
SELECT id FROM log WHERE purchases = '{"seller":"kathy"}'
We researched here and elsewhere for some hours ... it seems a bit more complex because the values are nested? We found e.g. some java or pgplsql implementations, but we are still hoping there is a "pure SQL" way. What would be a proper solution? Or should we re-organize our content like this:
id | purchases (json)
1 [{"product":"apple","price":5,"seller":"frank"},{"product":"bred","price":3,"seller":"kathy"}]
2 [{"product":"milk","price":3,"seller":"anne"},{"product":"banana","price":2,"seller":"frank"}]
3 [{"product":"bred","price":4,"seller":"kathy"}]
But what we found, this would be even more complex, because we have to explode the arrays within the query. Any short hint? Thanks!

Answer 1

Check json_each() and #>> Postgres JSON functions:
WITH log(id,purchases) AS ( VALUES
(1,'{"apple":{"price":5,"seller":"frank"}, "bred":{"price":3,"seller":"kathy"}}'::JSON),
(2,'{"milk":{"price":3,"seller":"anne"}, "banana":{"price":2,"seller":"frank"}}'::JSON),
(3,'{"bred":{"price":4,"seller":"kathy"}}'::JSON)
)
SELECT log.* FROM log,
json_each(log.purchases) as purchase
WHERE
purchase.value#>>'{seller}' = 'kathy';
Result:
id | purchases
----+-----------------------------------------------------------------------------
1 | {"apple":{"price":5,"seller":"frank"}, "bred":{"price":3,"seller":"kathy"}}
3 | {"bred":{"price":4,"seller":"kathy"}}
(2 rows)