How to make this sql with query array_agg? - sql

Question

I want to make a query
select * from projects where user_id = 3;
and depending on it's result r, I need to make n queries, where n is the length l of r. eg:
| id | project_name | description | user_id |
| 1 | Project A | lorem ipsu | 3 |
| 4 | Project B | lorem ipsu | 3 |
l => 2 then:
select * from images where project_id = 1;
select * from images where project_id = 4;
Ok, you can see where this is going if l is too big. Too many selects, too many access to the database. Is there a better way to achieve an end result like so:
| id | project_name | description | user_id | images |
| 1 | Project A | lorem ipsu | 3 | {imgX,imgY,imgZ} |
| 4 | Project B | lorem ipsu | 3 | {imgA,imgB} |
I heard about array_agg function on postgres. Maybe that's the answer? Anyways, these are my table descriptions:
Table "public.projects"
Column | Type | Modifiers
-------------+--------------------------+-------------------------------------------------------
id | integer | not null default nextval('projects_id_seq'::regclass)
name | character varying(255) |
description | character varying(255) |
user_id | integer |
created_at | timestamp with time zone |
updated_at | timestamp with time zone |
Table "public.images"
Column | Type | Modifiers
------------+--------------------------+-----------------------------------------------------
id | integer | not null default nextval('images_id_seq'::regclass)
name | character varying(255) |
url | character varying(255) |
project_id | integer |
created_at | timestamp with time zone |
updated_at | timestamp with time zone |
And thank you in advance :D

Answer 1

array_agg is like any other aggregate function (count, sum), but returns an array instead of a scalar value. What you need can be achieved simply by joining and grouping the 2 tables.
SELECT p.id, p.name, p.description, p.user_id, array_agg(i.name) images
FROM projects p
LEFT JOIN images i ON p.id = i.project_id
GROUP BY p.id, p.name, p.description, p.user_id

Answer 2

You want records from projects plus records in image names as array matching by project_id :
SELECT *
FROM projects
LEFT JOIN LATERAL (SELECT array_agg(name) AS images FROM images WHERE project_id = projects.project_id) x ON true
WHERE user_id = '3'
sqlfiddle

Answer 3

Probably the easiest solution for you is a sub-select. This comes closest to the individual SELECT statements that you mention earlier:
SELECT * FROM images
WHERE project_id IN (
SELECT project_id FROM projects
WHERE user_id = 3);