Re: Subquery to select max(date) value

On Fri, 15 Feb 2019, Andrew Gierth wrote:

> LATERAL (SELECT ...)   is syntactically like (SELECT ...) in that it comes
> _after_ a "," in the from-clause or after a [LEFT] JOIN keyword. Don't
> think of LATERAL as being a type of join, think of it as qualifying the
> (SELECT ...) that follows.

Andrew,

Thank you. Now I understand the difference.

> Rich>         from activities as a
> Rich>         where a.next_contact is not null and a.next_contact <= 'today' and
> Rich>               a.next_contact > '2018-12-31'
>
> You'd want a condition here that references the "people" table; the whole
> point of LATERAL is that it opens up the scope of column references in the
> subquery to include those tables which are to its left in the from-clause.

And here I got it backwards, thinking the subquery could reference the
columns in the people table from the initial select.

> Rich>         order by person_id,next_contact);
>
> and I'm guessing you want that ordered by next_contact alone, possibly
> with LIMIT 1 to get just the nearest following next_contact time.

That's true. With 'discrete on' only the most recent next_contact date is
returned.

More work over the weekend on this now I have a better understanding of
lateral.

Thanks again,

Rich