Q

Returning data when requested time period has no data, part 2

Here is the continuation of Jason's response. See part 1.

You might write the SQL like this:

select convert(varchar,Day,107) Day, 
       count(Passed) TotalCars
  from Days left join TollCars
    on TollCars.Passed >= Days.Day and 
       TollCars.Passed < Days.Day + 1
  where Color = 'RED'
  group by Day;
Here are the resulting rows:
Day          TotalCars
------------ ---------
Mar 01, 2002         1
Mar 04, 2002         1
Mar 07, 2002         2
But, you find that you have gaps in your days even though you've used an outer join. We need to remember that in an outer join situation, when no rows exist for the join condition in the non-preserved table, the columns are padded with nulls. So if the Color column is NULL, then the condition Color = ?RED' will not resolve to TRUE, and so the whole resulting row, including the preserved table's columns, is thrown out. The solution is to move the condition into the ON clause, making it part of the join condition. This has the same affect as filtering the TollCars table before performing the join. The following SQL statement will provide us with the results we want:
select convert(varchar,Day,107) Day, 
       count(Passed) TotalCars
  from Days left join TollCars
    on TollCars.Passed >= Days.Day and 
       TollCars.Passed < Days.Day + 1 and 
       Color = 'RED'
  group by Day;

Day          TotalCars
------------ ---------
Mar 01, 2002         1
Mar 02, 2002         0
Mar 03, 2002         0
Mar 04, 2002         1
Mar 05, 2002         0
Mar 06, 2002         0
Mar 07, 2002         2

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This was first published in March 2002

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