Join only when relation data exists - sql

Question

These are my tables.
CREATE TABLE IF NOT EXISTS categories (
category_id serial primary key,
category text not null,
user_id int not null
);
CREATE TABLE IF NOT EXISTS activities (
activity_id serial primary key,
activity text not null,
user_id int not null
);
CREATE TABLE IF NOT EXISTS categories_to_activities (
category_id int not null REFERENCES categories (category_id) ON UPDATE CASCADE ON DELETE CASCADE,
activity_id int not null REFERENCES activities (activity_id) ON UPDATE CASCADE ON DELETE CASCADE,
CONSTRAINT category_activity_pkey PRIMARY KEY (category_id, activity_id)
);
This is the query I'm using to get all activities with their categories.
SELECT a.*, ARRAY_AGG ( ROW_TO_JSON (c) ) categories
FROM activities a
JOIN categories_to_activities ca ON a.activity_id = ca.activity_id
JOIN categories c ON ca.category_id = c.category_id
WHERE a.user_id = ${userId}
GROUP BY a.activity_id
The issue is that if an activity does not have a category assigned, it won't get returned.
I'm having trouble combining JOIN with CASE, which I suppose is what I need. Essentially I want to JOIN only when there is some record in categories_to_activities?
How do I do that?

Answer 1

You could use a left join, and return nulls for activities without categories:
SELECT a.*,
CASE WHEN ca.activity_id IS NOT NULL THEN ARRAY_AGG(ROW_TO_JSON(c))
ELSE ARRAY[]::JSON[]
END as categories
FROM activities a
LEFT JOIN categories_to_activities ca ON a.activity_id = ca.activity_id
LEFT JOIN categories c ON ca.category_id = c.category_id
WHERE a.user_id = ${userId}
GROUP BY a.activity_id

Answer 2

What I ended up with, thanks to #Murenik
SELECT a.*,
CASE WHEN ca.activity_id IS NOT NULL
THEN ARRAY_AGG ( ROW_TO_JSON (c) )
ELSE ARRAY[]::JSON[]
END as categories
FROM activities a
LEFT JOIN categories_to_activities ca ON a.activity_id = ca.activity_id
LEFT JOIN categories c ON ca.category_id = c.category_id
WHERE a.user_id = ${userId}
GROUP BY a.activity_id, ca.activity_id