How can I get all keys from a JSON column in Postgres?

If I have a table with a column named json_stuff, and I have two rows with

{ "things": "stuff" } and { "more_things": "more_stuff" }

in their json_stuff column, what query can I make across the table to receive [ things, more_things ] as a result?

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最佳答案

Use this:

select jsonb_object_keys(json_stuff) from table;

(Or just json_object_keys if you're using just json.)

The PostgreSQL json documentation is quite good. Take a look.

And as it is stated in the documentation, the function only gets the outer most keys. So if the data is a nested json structure, the function will not return any of the deeper keys.

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WITH t(json_stuff) AS ( VALUES
('{"things": "stuff"}'::JSON),
('{"more_things": "more_stuff"}'::JSON)
)
SELECT array_agg(stuff.key) result
FROM t, json_each(t.json_stuff) stuff;
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Here is the example if you want to get the key list of each object:

select array_agg(json_keys),id from (
select json_object_keys(json_stuff) as json_keys,id from table) a group by a.id

Here id is the identifier or unique value of each row. If the row cannot be distinguished by identifier, maybe it's better to try PL/pgSQL.

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Insert json_column and table

select distinct(tableProps.props) from (
select jsonb_object_keys(<json_column>) as props from <table>
) as tableProps
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I wanted to get the amount of keys from a JSONB structure, so I'm doing something like this:

select into cur some_jsonb from mytable where foo = 'bar';
select into keys array_length(array_agg(k), 1) from jsonb_object_keys(cur) as k;

I feel it is a little bit wrong, but it works. It's unfortunate that we can't get an array directly from the json_object_keys() function. That would save us some code.

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Here's a solution that implements the same semantics as MySQL's JSON_KEYS(), which...:

  • is NULL safe (i.e. when the array is empty, it produces [], not NULL, or an empty result set)
  • produces a JSON array, which is what I would have expected from how the question was phrased.
SELECT
o,
(
SELECT coalesce(json_agg(j), json_build_array())
FROM json_object_keys(o) AS j (j)
)
FROM (
VALUES ('{}'::json), ('{"a":1}'::json), ('{"a":1,"b":2}'::json)
) AS t (o)

Replace json by jsonb if needed.

Producing:

|o            |coalesce  |
|-------------|----------|
|{}           |[]        |
|{"a":1}      |["a"]     |
|{"a":1,"b":2}|["a", "b"]|

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