PostgreSQL joining using JSONB I have this SQL:

CREATE TABLE test(id SERIAL PRIMARY KEY, data JSONB);

INSERT INTO test(data) VALUES
   ('{"parent":null,"children":[2,3]}'),
   ('{"parent":1,   "children":[4,5]}'),
   ('{"parent":1,   "children":[]}'),
   ('{"parent":2,   "children":[]}'),
   ('{"parent":2,   "children":[]}');


That would give:

 id |                 data                 
----+--------------------------------------
  1 | {"parent": null, "children": [2, 3]}
  2 | {"parent": 1, "children": [4, 5]}
  3 | {"parent": 1, "children": []}
  4 | {"parent": 2, "children": []}
  5 | {"parent": 2, "children": []}


When doing normal one to many, it would show something like this:

SELECT * 
FROM test x1
  LEFT JOIN test x2
    ON x1.id = (x2.data-&gt;&gt;'parent')::INT;
 id |                 data                 | id |               data                
----+--------------------------------------+----+-----------------------------------
  1 | {"parent": null, "children": [2, 3]} |  2 | {"parent": 1, "children": [4, 5]}
  1 | {"parent": null, "children": [2, 3]} |  3 | {"parent": 1, "children": []}
  2 | {"parent": 1, "children": [4, 5]}    |  4 | {"parent": 2, "children": []}
  2 | {"parent": 1, "children": [4, 5]}    |  5 | {"parent": 2, "children": []}
  5 | {"parent": 2, "children": []}        |    | 
  4 | {"parent": 2, "children": []}        |    | 
  3 | {"parent": 1, "children": []}        |    | 


How to join based on children (using LEFT JOIN or WHERE IN)? I've tried:

SELECT data-&gt;&gt;'children' FROM test;
 ?column? 
----------
 [2, 3]
 [4, 5]
 []
 []
 []

SELECT json_array_elements((data-&gt;&gt;'children')::TEXT) FROM t...
               ^
HINT:  No function matches the given name and argument types. You might need to add explicit type casts.

SELECT json_array_elements((data-&gt;&gt;'children')::JSONB) FROM ...
               ^
HINT:  No function matches the given name and argument types. You might need to add explicit type casts.

SELECT json_to_record((data-&gt;&gt;'children')::JSON) FROM test;
ERROR:  function returning record called in context that cannot accept type record
HINT:  Try calling the function in the FROM clause using a column definition list.

SELECT * FROM json_to_record((test.data-&gt;&gt;'children')::JSON);
ERROR:  missing FROM-clause entry for table "test"
LINE 1: SELECT * FROM json_to_record((test.data-&gt;&gt;'children')::JSON)...
This would be more efficient:
With jsonb and jsonb_array_elements_text() in pg 9.4+
EXPLAIN 
SELECT p.id AS p_id, p.data
     , c.id AS c_id, c.data
FROM   test p
LEFT   JOIN LATERAL jsonb_array_elements_text(p.data->'children') pc(child) ON TRUE
LEFT   JOIN test c ON c.id = pc.child::int;
db&lt;&gt;fiddle here
About jsonb_array_elements_text():

How to turn JSON array into Postgres array?

Use the -&gt; operator instead of -&gt;&gt; in the reference to children. The way you have it, you'd first cast json / jsonb  to text and then back to json.
The clean way to call a set-returning function is LEFT [OUTER] JOIN LATERAL. This includes rows without children.
To exclude those, change to a [INNER] JOIN LATERAL or CROSS JOIN - or the shorthand syntax with a comma:
, json_array_elements(p.data-&gt;'children') pc(child)

Avoiding duplicate column names in result.
With json and json_array_elements() in pg 9.3
SELECT p.id AS p_id, p.data AS p_data
     , c.id AS c_id, c.data AS c_data
FROM   test p
LEFT   JOIN LATERAL json_array_elements(p.data-&gt;'children') pc(child) ON TRUE
LEFT   JOIN test c ON c.id = pc.child::text::int;

Old sqlfiddle
Aside: A normalized DB design with basic data types would be way more efficient for this.