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From: David G. Johnston <[email protected]>
To: Rich Shepard <[email protected]>
Cc: [email protected] <[email protected]>
Subject: Re: Extract only maximum date from column
Date: Thu, 4 Dec 2025 15:13:11 -0700
Message-ID: <CAKFQuwbzjjBi7YV1gSn=2jX7h-FK8kf8fbpWmpYqX-8pSD7beA@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<CAKFQuwZEWkgQeWSB7nqzBjHS8kP7kvouT6jhKvfe_G2e3cJvOQ@mail.gmail.com>
	<[email protected]>

On Thursday, December 4, 2025, Rich Shepard <[email protected]>
wrote:

> On Thu, 4 Dec 2025, David G. Johnston wrote:
>
> I would go with a lateral join subquery of the contracts table. Using an
>> aggregates to perform ranking is an anti-pattern. You want the contract
>> ranked first when ordered by contract_date. Either use a window function
>> to explicitly rank the contracts or use a limit/fetch clause to simply
>> return the first ordered one.
>>
>
> David,
>
> I'm closer, but still missing the proper syntax:
>
> select p.person_nbr, p.company_nbr, c.next_contact
> from people as p, contacts as c
> join lateral (select max(c.next_contact) as last_contact
>      where p.person_nbr = c.person_nbr and
>      last_contact >= '2025-11-01'
>      )
>      c on true;
>
> resulting in:
> psql:companies-contacted-2025.sql:9: ERROR:  aggregate functions are not
> allowed in FROM clause of their own query level
> LINE 3: join lateral (select max(c.next_contact) as last_contact
>

As mentioned, the aggregate max should be avoided - you aren’t doing
statistics, you are ranking.

Select person.*, lastcontact.* from person join lateral (select contact.*
from contact where contact.person_id=person.person_id  order by
last_contact_date desc limit 1) as lastcontact on true;

David J.

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