With a query like this
SELECT a.id, a.name,
COALESCE( json_agg(b.*), '[]'::json ),
COALESCE( json_agg(c.*), '[]'::json ),
FROM a
LEFT JOIN b ON a.id = b.a_id
LEFT JOIN c ON a.id = c.a_id
GROUP BY a.id, a.name;
When executed, both c
and b
will be multiplied by each other and produce duplicated entries in the JSON array object.
I tried changing the query to use 2 subqueries instead, but I get all sorts of errors and warning, such as "Subquery must return only one column", etc.
I also tried using LEFT OUTER JOIN
, but I guess I don't yet master how joining tables work as it only applies to b
and c
is still multiplied and contain duplicates.
Edit: using DISTINCT
errors with "could not identify an equality operator for type json" on the COALESCE
functions.
How can I fix this query and only aggregate unique rows?
Edit 2
I need to specify that both table b
and c
are actually VIEW
s, and they both have at least one json_agg
column as well, so I cannot just use json_agg(DISTINCT b.*)
. This would've been way too easy.
Edit 3
Here is a small snippet to reproduce the issue :
--DROP TABLE IF EXISTS tbl_a CASCADE;
--DROP TABLE IF EXISTS tbl_b CASCADE;
--DROP TABLE IF EXISTS tbl_c CASCADE;
CREATE TABLE tbl_a (
id bigserial NOT NULL,
name character varying(16),
CONSTRAINT "PK_tbl_a" PRIMARY KEY (id)
) WITH ( OIDS=FALSE );
CREATE TABLE tbl_b (
a_id bigint NOT NULL,
foo json NOT NULL DEFAULT '{}'::json,
CONSTRAINT "FK_tbl_b_a" FOREIGN KEY (a_id)
REFERENCES tbl_a (id) MATCH SIMPLE
ON UPDATE CASCADE ON DELETE CASCADE
) WITH ( OIDS=FALSE );
CREATE TABLE tbl_c (
a_id bigint NOT NULL,
bar json NOT NULL DEFAULT '{}'::json,
CONSTRAINT "FK_tbl_c_a" FOREIGN KEY (a_id)
REFERENCES tbl_a (id) MATCH SIMPLE
ON UPDATE CASCADE ON DELETE CASCADE
) WITH ( OIDS=FALSE );
INSERT INTO tbl_a (id,name) VALUES (1, 'Test');
INSERT INTO tbl_b (a_id, foo) VALUES (1, '{"foo":"Hello"}'::json), (1, '{"foo":"World"}'::json);
INSERT INTO tbl_c (a_id, bar) VALUES (1, '{"bar":"abc"}'::json), (1, '{"bar":"def"}'::json);
SELECT tbl_a.id, tbl_a.name,
COALESCE(json_agg(tbl_b.*), '{}'::json),
COALESCE(json_agg(tbl_c.*), '{}'::json)
FROM tbl_a
LEFT JOIN tbl_b ON tbl_a.id = tbl_b.a_id
LEFT JOIN tbl_c ON tbl_a.id = tbl_c.a_id
GROUP BY tbl_a.id, tbl_a.name;
Returns
id name coalesce coalesce
-- ------ ------------------------------------ ----------------------
1 "Test" "[{"a_id":1,"foo":{"foo":"World"}}, "[{"a_id":1,"bar":{"bar":"abc"}},
{"a_id":1,"foo":{"foo":"Hello"}}, {"a_id":1,"bar":{"bar":"abc"}},
{"a_id":1,"foo":{"foo":"World"}}, {"a_id":1,"bar":{"bar":"def"}},
{"a_id":1,"foo":{"foo":"Hello"}}]" {"a_id":1,"bar":{"bar":"def"}}]"
Best Answer
I have found a solution. I am not sure if it is optimal, but it works.
Which correctly returns