Re: Refining query statement

On Tue, 15 Jan 2019, Thomas Kellerer wrote:

> With regards to "cleaner": the first thing to do is to remove the
> parentheses around the column list. In Postgres "(a,b,c)" creates a single
> column with an anonymous record type (that contains three fields), rather
> than selecting three columns. In other DBMS those parentheses are simply
> useless.

Thomas,

I should have seen that myself. Thanks for pointing out the obvious to me.

> "cleaner" is always subjective, but I find explicit JOIN operators a lot
> cleaner than the old implicit joins.

   This does make sense; I need to refresh my knowledge of JOIN operators and
will do so.

> The condition "A.next_contact is not null" is actually no necessary
> because you already have a condition on that column, so NULL values won't
> be returned anyway.

   True that.

> To get the "most recent one" in Postgres, DISTINCT ON () is usually the
> best way to do it: So we end up with something like this:
>
>    select distinct on (C.contact_id) C.contact_id, C.lname, C.fname, C.direct_phone, O.org_name, A.next_contact
>    from Contacts as C
>      join Organizations as O on C.org_id = O.org_id
>      join Activities as A on C.contact_id = A.contact_id
>    where A.next_contact <= 'today'
>      and A.next_contact > '2018-12-31'
>    order by c.contact_id, a.next_contact DESC;

   This insight really helps. Now I know how to approach other queries.

Many thanks,

Rich