Re: Refining query statement

Rich Shepard schrieb am 15.01.2019 um 16:39:
>   Working with my sales/client management system using psql I have a select
> statement to identify contacts to be made. This statement works:
> 
> select (C.contact_id, C.lname, C.fname, C.direct_phone, O.org_name, A.next_contact)
> from Contacts as C, Organizations as O, Activities as A
> where C.org_id = O.org_id and C.contact_id = A.contact_id and
>       A.next_contact <= 'today' and A.next_contact > '2018-12-31' and
>       A.next_contact is not null;
> 
> but would benefit from tweaking. When I have had multiple contacts with
> someone I want only the most recent one displayed, not all, and they should
> be no more ancient than a defined period (e.g., a month).
> 
>   I want to learn how to make this query cleaner and more flexible. When I
> write the UI for this I want to be able to specify a data range in addition
> to a fixed 'today'. Pointers on what to read will be very helpful.

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
selectingthree columns.
 
In other DBMS those parentheses are simply useless.

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

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. 
 

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;