PostgreSQL , Select from 2 tables, but only the latest element from table 2

Question

Hey, I have 2 tables in PostgreSql:

1 - documents: id, title
2 - updates: id, document_id, date

and some data:

documents:

Answer 1

Use PostgreSQL extension DISTINCT ON:

SELECT  DISTINCT ON (documents.id) *
FROM    document
JOIN    updates
ON      updates.document_id = document_id
ORDER BY
        documents.id, updates.date DESC

This will take the first row from each document.id cluster in ORDER BY order.

Test script to check:

SELECT  DISTINCT ON (documents.id) *
FROM    (
        VALUES
        (1, 'Test Title'),
        (2, 'Test Title 2')
        ) documents (id, title)
JOIN    (
        VALUES
        (1, 1, '2006-01-01'::DATE),
        (2, 1, '2007-01-01'::DATE),
        (3, 1, '2008-01-01'::DATE),
        (4, 2, '2009-01-01'::DATE),
        (5, 2, '2010-01-01'::DATE)
        ) updates (id, document_id, date)
ON      updates.document_id = documents.id
ORDER BY
        documents.id, updates.date DESC