Show data from table even if there is no data!! Oracle

Question

I have a query which shows count of messages received based on dates. For Eg:

1 | 1-May-2012 
3 | 3-May-2012 
4 | 6-May-2012 
7 | 7-May-2012 
9 | 9-May-2012 
5 |         


        

Answer 1

First, it sounds like your application would benefit from a calendar table. A calendar table is a list of dates and information about the dates.

Second, you can do this without using temporary tables. Here is the approach:

with constants as (select min(thedate>) as firstdate from )
     dates as (select(  + rownum - 1) as thedate
               from (select rownum
                     from  cross join constants
                     where rownum < sysdate -  + 1
                    ) seq
              )
select dates.thedate, count(t.date)
from dates left outer join
      t
     on t.date = dates.thedate
group by dates.thedate
Here is the idea.  The alias constants records the earliest date in your table.  The alias dates then creates a sequence of dates.  The inner subquery calculates a sequence of integers, using rownum, and then adds these to the first date.  Note this assumes that you have on average at least one transaction per date.  If not, you can use a bigger table.
The final part is the join that is used to bring back information about the dates.  Note the use of count(t.date) instead of count(*).  This counts the number of records in your table, which should be 0 for dates with no data.

                                                        

              
                
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