How to join only one row in joined table with postgres?

Question

I have the following schema:

CREATE TABLE author (
    id   integer
  , name varchar(255)
);
CREATE TABLE book (
    id        integer
  , author_id integer
  ,          


        

Answer 1

I've done something similar for a chat system, where room holds the metadata and list contains the messages. I ended up using the Postgresql LATERAL JOIN which worked like a charm.

SELECT MR.id AS room_id, MR.created_at AS room_created, 
    lastmess.content as lastmessage_content, lastmess.datetime as lastmessage_when
FROM message.room MR
    LEFT JOIN LATERAL (
        SELECT content, datetime
        FROM message.list
        WHERE room_id = MR.id
        ORDER BY datetime DESC 
        LIMIT 1) lastmess ON true
ORDER BY lastmessage_when DESC NULLS LAST, MR.created_at DESC

For more info see https://heap.io/blog/engineering/postgresqls-powerful-new-join-type-lateral