Postgres: left join with order by and limit 1

duplicates, foreign-key-relationship, left-join, postgresql, sql-order-by

Solution

I managed to solve it using Windows Function:

WITH ranked_relationship AS(
    SELECT
        *
        ,row_number() OVER (PARTITION BY fk_company ORDER BY dt_start DESC) as dt_last_addr
    FROM relationship
)

SELECT
    company.*
    address.*,
    dt_last_addr as dt_relationship
FROM
    company
    LEFT JOIN ranked_relationship as relationship
            ON relationship.fk_company = company.pk_company AND dt_last_addr = 1
    LEFT JOIN address ON address.pk_address = relationship.fk_address

row_number() creates an int counter for each record, inside each window based to fk_company. For each window, the record with latest date comes first with rank 1, then `dt_last_addr = 1` makes sure the JOIN happens only once for each `fk_company`, with the record with latest address.

Window Functions are very powerful and few ppl use them, they avoid many complex joins and subqueries!

Problem

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 Therefore, the removal of outdated data must be done (AFAIK) in LEFT JOIN's ON clause.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?

Original source