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Subject: Re: Extract only maximum date from column
From: Alban Hertroys <haramrae@gmail.com>
In-Reply-To: <db81f086-e1a5-5798-f9bd-5bed5b1bc7f4@appl-ecosys.com>
Date: Thu, 4 Dec 2025 23:58:17 +0100
Cc: pgsql-general@postgresql.org
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Message-Id: <BE035292-C9E3-4A46-9DD1-7026A16292DF@gmail.com>
References: <db81f086-e1a5-5798-f9bd-5bed5b1bc7f4@appl-ecosys.com>
To: Rich Shepard <rshepard@appl-ecosys.com>
Archived-At: <https://www.postgresql.org/message-id/BE035292-C9E3-4A46-9DD1-7026A16292DF%40gmail.com>
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> On 4 Dec 2025, at 20:55, Rich Shepard <rshepard@appl-ecosys.com> =
wrote:
>=20
> I want the script to extract only the maximum `next_contact' date and
> haven't learned how to do this from my reading of web sites.
>=20
> The script:
>=20
> select p.person_nbr, p.company_nbr, c.next_contact
> from people as p, contacts as c
> where c.next_contact >=3D '2025-11-01'
> group by p.person_nbr, p.company_nbr, c.next_contact
> order by p.person_nbr, p.company_nbr, max(c.next_contact);
>=20
> returns all contacts rather than only the latest one.
>=20
> Is using a sub-select the proper way?
>=20
> TIA,
>=20
> Rich

That looks like a classical case for a correlated subquery with WHERE =
NOT EXISTS.

Something like:

select p.person_nbr, p.company_nbr, c.next_contact
from people as p
join contacts as c
on -- I=E2=80=99m really missing some kind of relation between p and c =
here, I think that=E2=80=99s relevant
where c.next_contact >=3D =E2=80=982025-11-01=E2=80=99
-- make sure there=E2=80=99s no later contact
and not exists (
	select 1 -- the value is irrelevant, as long as there=E2=80=99s =
no later instance of a contact
	from contacts c2
	where c2.next_contact >=3D =E2=80=982025-11-01=E2=80=99
	and c2.next_contact > c.next_contact
)

P.S. My mail-client tried to outsmart me in this reply (in no small =
part) and I just got back from the pub, so I can=E2=80=99t exactly =
guarantee correctness of the above, but the principal idea should be =
solid.

Alban Hertroys
--
There is always an exception to always.