List all months with a total regardless of null - sql

Question

I have a very small SQL table that lists courses attended and the date of attendance. I can use the code below to count the attendees for each month
select to_char(DATE_ATTENDED,'YYYY/MM'),
COUNT (*)
FROM TRAINING_COURSE_ATTENDED
WHERE COURSE_ATTENDED = 'Fire Safety'
GROUP BY to_char(DATE_ATTENDED,'YYYY/MM')
ORDER BY to_char(DATE_ATTENDED,'YYYY/MM')
This returns a list as expected for each month that has attendees. However I would like to list it as
January 2
February 0
March 5
How do I show the count results along with the nulls? My table is very basic
1234 01-JAN-15 Fire Safety
108 01-JAN-15 Fire Safety
1443 02-DEC-15 Healthcare
1388 03-FEB-15 Emergency
1355 06-MAR-15 Fire Safety
1322 09-SEP-15 Fire Safety
1234 11-DEC-15 Fire Safety
I just need to display each month and the total attendees for Fire Safety only. Not used SQL developer for a while so any help appreciated.

Answer 1

You would need a calendar table to select a period you want to display. Simplified code would look like this:
select to_char(c.Date_dt,'YYYY/MM')
, COUNT (*)
FROM calendar as c
left join TRAINING_COURSE_ATTENDED as tca
on tca.DATE_ATTENDED = c.Date_dt
WHERE tca.COURSE_ATTENDED = 'Fire Safety'
and c.Date_dt between [period_start_dt] and [period_end_dt]
GROUP BY to_char(c.Date_dt,'YYYY/MM')
ORDER BY to_char(c.Date_dt,'YYYY/MM')

Answer 2

You can create your own set required year month's on-fly with 0 count and use query as below.
Select yrmth,sum(counter) from
(
select to_char(date_attended,'YYYYMM') yrmth,
COUNT (1) counter
From TRAINING_COURSE_ATTENDED Where COURSE_ATTENDED = 'Fire Safety'
Group By Y to_char(date_attended,'YYYYMM')
Union All
Select To_Char(2015||Lpad(Rownum,2,0)),0 from Dual Connect By Rownum <= 12
)
group by yrmth
order by 1
If you want to show multiple year's, just change the 2nd query to
Select To_Char(Year||Lpad(Month,2,0)) , 0
From
(select Rownum Month from Dual Connect By Rownum <= 12),
(select 2015+Rownum-1 Year from Dual Connect By Rownum <= 3)

Answer 3

Try this :
SELECT Trunc(date_attended, 'MM') Month,
Sum(CASE
WHEN course_attended = 'Fire Safety' THEN 1
ELSE 0
END) Fire_Safety
FROM training_course_attended
GROUP BY Trunc(date_attended, 'MM')
ORDER BY Trunc(date_attended, 'MM')

Answer 4

Another way to generate a calendar table inline:
with calendar (month_start, month_end) as
( select add_months(date '2014-12-01', rownum)
, add_months(date '2014-12-01', rownum +1) - interval '1' second
from dual
connect by rownum <= 12 )
select to_char(c.month_start,'YYYY/MM') as course_month
, count(tca.course_attended) as attended
from calendar c
left join training_course_attended tca
on tca.date_attended between c.month_start and c.month_end
and tca.course_attended = 'Fire Safety'
group by to_char(c.month_start,'YYYY/MM')
order by 1;
(You could also have only the month start in the calendar table, and join on trunc(tca.date_attended,'MONTH') = c.month_start, though if you had indexes or partitioning on tca.date_attended that might be less efficient.)