Postgres: left join with order by and limit 1

Question

I have the situation:

Table1 has a list of companies.
Table2 has a list of addresses.
Table3 is a N relationship of Table1 and Table2, with fields 'begin' and 'end'.

Because companies may move over time, a LEFT JOIN among them results in multiple records for each company.

begin and end fields are never NULL. The solution to find the latest address is use a ORDER BY being DESC, and to remove older addresses is a LIMIT 1.

That works fine if the query can bring only 1 company. But I need a query that brings all Table1 records, joined with their current Table2 addresses. Therefore, the removal of outdated data must be done (AFAIK) in LEFT JOIN's ON clause.

Any idea how I can build the clause to not create duplicated Table1 companies and bring latest address?

Answer 1

Use a dependent subquery with max() function in a join condition.
Something like in this example:

SELECT *
FROM companies c
LEFT JOIN relationship r
ON c.company_id = r.company_id
   AND r."begin" = (
        SELECT max("begin")
        FROM relationship r1
        WHERE c.company_id = r1.company_id
     )
INNER JOIN addresses a
ON a.address_id = r.address_id 

demo: http://sqlfiddle.com/#!15/f80c6/2