Select count of rows in another table in a Postgres SELECT statement

Question

I don\'t know quite how to phrase this so please help me with the title as well. :)

I have two tables. Let\'s call them A and B. The

Answer 1

Accepted answer is inefficient (slow) based on my tests. The subquery of table B executing for every row of table A. I'm using following approach based on grouping and joining. It works much faster:

SELECT A.id, QTY.quantity FROM A
LEFT JOIN
    (SELECT COUNT(B.a_id) AS quantity, B.a_id FROM B GROUP BY B.a_id) AS QTY
ON A.id = QTY.a_id

Another variant:

SELECT A.id, COUNT(B.a_id) AS quantity FROM A
LEFT JOIN B ON B.a_id = A.id
GROUP BY A.id