Re: Selecting the most recent timestamptz

On Thu, Feb 22, 2018 at 1:09 AM, Alexander Farber <
alexander(dot)farber(at)gmail(dot)com> wrote:

> Good morning, there are these 2 records in a table:
>
> # select m.played, m.mid, m.action, m.gid, m.uid from words_moves m where
> gid=10;
> played | mid | action | gid | uid
> -------------------------------+-----+--------+-----+-----
> 2018-02-19 14:42:08.46222+01 | 12 | play | 10 | 9
> 2018-02-20 15:06:01.430634+01 | 216 | expire | 10 | 11
> (2 rows)
>
> I try to get the record with the latest timestamp by adding a NOT EXISTS
> condition -
>
> # select m.played, m.mid, m.action, m.gid, m.uid from words_moves m where
> gid=10 and not exists (select 1 from words_moves x where m.mid=x.mid AND
> x.played > m.played);
> played | mid | action | gid | uid
> -------------------------------+-----+--------+-----+-----
> 2018-02-19 14:42:08.46222+01 | 12 | play | 10 | 9
> 2018-02-20 15:06:01.430634+01 | 216 | expire | 10 | 11
> (2 rows)
>
> Why are still 2 records returned? I am probably overlooking something
> simple, sorry...
>
> Thank you
> Alex
>
>
In your example, you have different values for mid. I'm thinking you meant
gid?

select m.played, m.mid, m.action, m.gid, m.uid from words_moves m where
gid=10 and not exists (select 1 from words_moves x where *m.gid=x.gid* AND
x.played > m.played);

On a related note for the list, I know of at least two other ways to do
this. Are any of them better and worse?

SELECT DISTINCT ON (gid) [fields] FROM words_moves m WHERE gid=10 ORDER BY
gid,played DESC
SELECT [fields] FROM words_moves m WHERE gid=10 ORDER BY played DESC limit
1;

Cheers,
Ken

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