SQL query to return nil for dates not present in the table - sql

Question

I have a table 'my_table'. It has the following data :
ID --- Date
1 --- 01/30/2012
2 --- 01/30/2012
3 --- 05/30/2012
I can write a SQL query to return the count of ID's between certain dates, grouped by month, like this :
{"01/30/2012" => 2, "05/30/2012" => 1}
How can I get a result which has all the missing months between the requested dates with value '0', like this :
{"01/30/2012" => 2, "02/30/2012" => 0, "03/30/2012" => 0, "04/30/2012" => 0, "05/30/2012" => 1}
Thanks in advance.

Answer 1

The way I do it is to have a static table with list of all the dates. In your case that's 30th of each month (what about February?). Lets call this table REF_DATE. It has a single column DT that holds the date.
Assuming that my_table only contains 0 or at most 1 distinct date (30th) in each month, what you need to do is:
select DT,count(ID) from
REF_DT REF
left outer join my_table MT
on REF.DT=my_table.DATE
group by REF.DT;

Answer 2

I came up with somewhat hackish way through rails
class Month<Date # for getting months in range
def succ
self >> 1
end
end
range = Month.new(2010,1,1)..Month.new(2013,1,1) # range of date to query
months=Hash.new
(range).each do |month|
months.merge!({month.to_s => 0}) # get all months as per range requirement of project
end
db_months = MyTable.find_all_by_date(range).group_by{ |u| u.date.beginning_of_month }.map{|m,v| [m.to_s , v.size]} #get all records grouped by months
all_months = months.merge(Hash[db_months]) # merge all missing months
Replace the range with the dates you want also the format of the date as per your requirement.