Can I get a whole year one table or do I have to change the query for each month? - sql

Question

The following code is probably self explanatory
Lists number of active subscribers 1st of june
select
count(subscribers) as activeSubscribers
from subscriberlist sub
where
to_date('2012-06', 'yyyy-mm')
between sub.firstDayOfSubscription and sub.lastDayOfSubscription
But if I want to get the number of subscribers in the beginning of every month the past year, what could I do (besides changing the date in the query and repeating).

Answer 1

You can generate a list of months as follows:
SELECT add_months (trunc (to_date('01/01/2012','MM/DD/YYYY'), 'MM'), 1*LEVEL -1) FirstDay
FROM Dual
CONNECT BY LEVEL <= MONTHS_BETWEEN(to_date('12/31/2012','MM/DD/YYYY'), to_date('01/01/2012','MM/DD/YYYY')) + 1
ORDER BY FirstDay
This would give you the following output:
FIRSTDAY
--------
January, 01 2012 00:00:00+0000
February, 01 2012 00:00:00+0000
March, 01 2012 00:00:00+0000
April, 01 2012 00:00:00+0000
May, 01 2012 00:00:00+0000
June, 01 2012 00:00:00+0000
July, 01 2012 00:00:00+0000
August, 01 2012 00:00:00+0000
September, 01 2012 00:00:00+0000
October, 01 2012 00:00:00+0000
November, 01 2012 00:00:00+0000
December, 01 2012 00:00:00+0000
You can then include that query as an inline view as follows:
SELECT months.FirstDay,
(SELECT count(subscribers)
FROM subscriberlist sub
WHERE months.FirstDay BETWEEN sub.firstDayOfSubscription AND sub.lastDayOfSubscription
) AS activeSubscribers
FROM (SELECT add_months(trunc (to_date('01/01/2012','MM/DD/YYYY'), 'MM'), 1*LEVEL -1) FirstDay
FROM Dual
CONNECT BY LEVEL <= MONTHS_BETWEEN(to_date('12/31/2012','MM/DD/YYYY'), to_date('01/01/2012','MM/DD/YYYY')) + 1
ORDER BY FirstDay) Months
I'd probably wrap this in a sproc that accepts #startDate and #endDate parameters to define the date range you're interested, but I followed your syntax with the date range being defined with strings.
I was playing with this using this SqlFiddle

Answer 2

This might be a good use for a date lookup table. I've found them useful enough to try to always have one in any decent-sized database.
Build a table in your database with one line per day, for a reasonable period of time... say from ten years ago to ten years from now (depending on what sort of queries you run).
In the table add a number of fields based on the date:
Date
Week
Week_Ending
Month
Month_DayOf
Month_Beginning
Year
Business_Day
Weekend
Holiday
etc...
The handy thing about this is that once it's populated, you can join it to other tables by the date field to summarize or filter data by date however you need. For example:
select
dt.Month
count(subscribers) as activeSubscribers
from subscriberlist sub
join (select unique datelookup.Month_Beginning
from date_reference ) dt
on dt.Month_Beginning between sub.firstDayOfSubscription and sub.lastDayOfSubscription
group by
dt.Month;
(Off the top of my head, I'm not sure if this syntax is exactly right, especially for Oracle.)
If you don't have the ability to add one to your database, you can create a temporary table as part of your query and fill it with the date ranges you're concerned with.