Grouping by timestamp, how to return 0 when there's no record

On 19 September 2011 12:01, Koen Van Impe <koen.vanimpe@belnet.be> wrote:

Hello,

I'm trying to write a query that groups records by hour.
This works fine but when there are no records for a specific hour the
query does not return a result (this seems 'logic') and I'd like it to
return '0'. I suspect I should play around with 'interval' or something
but I can't get it to work.

If you're working with an interval of time that's known when issuing the
query, you can add the missing hours by joining your results with
generate_series().

If not, you'll probably have to create a table with enough dates and hours
to join against, for which generate_series() is once again quite
convenient. I'd like to see a resultset similar to this

It works like this:
select hours.h from generate_series(0, 23) AS hours(h);

Alban.
--
If you can't see the forest for the trees,
Cut the trees and you'll see there is no forest.

Hi, this may be a start:

-- This will make our day better :)
with base_query (tstmp) as (
select DATE_TRUNC('hour',timestamp) as tstmp
FROM record
WHERE
record.timestamp BETWEEN ( CURRENT_TIMESTAMP + INTERVAL '-7 day') and
(CURRENT_TIMESTAMP) -- this I don't understand
)
-- The following will make the same as your query
select count(*) as qt, tstmp as dfilter from base_query
-- And this adds the zero's
union
select 0 as qt, min(tstmp) + interval generate_series( 0, max(tstmp) -
min(tstmp) ) || ' hours' as dfilter
from base_query
where dfilter not in tstmp; -- Only for those who don't!

Don't forget to tell us how you ended up!