Re: Window function?

On Sat, Jun 4, 2022 at 7:50 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Robert Stanford <rstanford(at)gmail(dot)com> writes:
> > Can anyone suggest an elegant way to coalesce consecutive rows so only
> the
> > first start time and last end time for each group of events (by input) is
> > returned.
>
> Window functions don't change the number of query result rows, so that
> idea is a dead end. The standard way to solve this is GROUP BY:
>
> SELECT Input, MIN(Start) AS Start, MAX(End) AS End
> FROM ...
> GROUP BY Input
> ORDER BY Input -- optional
>

Assuming that input group 4 really is supposed to show up twice in the
output then the final solution is going to have to be a combination of this
and window functions. You need the later in order to be able to say "input
4, appearance 1" and "input 4, appearance 2" - which lead/lag tends to help
with. Then the group by becomes {input, appearance}.

David J.