Select table as json array - sql

Question

I have three tables in PostgreSQL: A, B, C.
I want to get a row from table A with a specific id, plus all records from tables B and C with matching id as aggregated JSON.
For example:
Table A Table B Table C
---------------------------------------------------------------
id / colum1 / colum2 id/ colum 1 id / column1
1 someValue, somValue 1 someVal1 1 someVal1
1 someVal2 1 someVal2
The expected output for id = 1 would be:
a.column1 a.column2 ARRAY_JSON_B ARRAY_JSON_C
------------------------------------------------------------------------------
someValue someValue [{colum1:'someVal1'}, [{colum1:'someVal1'},
{colum1:'someVal2'}] {colum1:'someVal2'}]

Answer 1

This requires Postgres 9.3 or later.
Simple case
I suggest to use the simpler json_agg() that's meant for this purpose, in LATERAL joins:
SELECT *
FROM a
LEFT JOIN LATERAL (SELECT json_agg(b) AS array_json_b FROM b WHERE id = a.id) b ON true
LEFT JOIN LATERAL (SELECT json_agg(c) AS array_json_c FROM c WHERE id = a.id) c ON true
WHERE id = 1;
LEFT JOIN LATERAL ... ON true keeps rows in the result that have no match on the left side of the join. Details:
What is the difference between LATERAL JOIN and a subquery in PostgreSQL?
Subtle difference: This query returns NULL where no match is found in b or c, #stas' query with correlated subqueries returns an empty array instead. May or may not be important.
Actual answer
Your example in the question excludes the redundant id column in b and c from the result - which makes sense. To achieve this, you can't use #stas' simple correlated subquery. While it would still work for a single column instead of the whole row, it would lose the column name and produce a simple array. Also, it would not work for more than one column.
Use json_object_agg() for a single selected column (which also allows to chose the tag name freely):
SELECT *
FROM a
LEFT JOIN LATERAL (
SELECT json_object_agg('colum1', colum1) AS array_json_b
FROM b WHERE id = a.id
) b ON true
LEFT JOIN LATERAL (
SELECT json_object_agg('colum1', colum1) AS array_json_c
FROM c WHERE id = a.id
) c ON true
WHERE id = 1;
Or use a subselect for any selection (col1 and col2 in this example):
SELECT *
FROM a
LEFT JOIN LATERAL (
SELECT json_agg(x) AS array_json_b
FROM (SELECT col1, col2 FROM b WHERE id = a.id) x
) b ON true
LEFT JOIN LATERAL (
SELECT json_agg(x) AS array_json_c
FROM (SELECT col1, col2 FROM c WHERE id = a.id) x
) c ON true
WHERE id = 1;
Related:
Return multiple columns of the same row as JSON array of objects
How do I return a jsonb array and array of objects from my data?

Answer 2

select
a.*,
to_json(array(select b from b where b.id = a.id)) array_json_b,
to_json(array(select c from c where c.id = a.id)) array_json_c
from a
where
a.id = 1;
I hope your Postgresql version is 9.3 or higher. There is a clever function to_json which can convert anything to json. So we take an array of all related rows from b and convert it. Same with c.