On 1/15/19 9:47 AM, Thomas Kellerer wrote:
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 selecting three 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;
And I've never liked this method (though I'm old and crotchety)....
--
Angular momentum makes the world go 'round.
