SQL of a daterange without empty spaces

Question

I need to generate a diagram out of data from a table. This table has the following date:

Timestamp | Value
01-20-2013| 5
01-21-2013| 7
01-22-2013| 3
01-25-2013| 5

As you can see not every date has a value. If I put that into a diagram it looks weird. Dates are used for the X-axis. As 01-23-2013 and 01-24-2013 is missing this values are either not printed in the diagram (looks weird) or the are printed put the line of the diagram goes from 3 directly to 5 and not to 0 as it should.

Is there a way via SQL to select the data so that it looks like this:

Timestamp | Value
01-20-2013| 5
01-21-2013| 7
01-22-2013| 3
01-23-2013| 0
01-24-2013| 0
01-25-2013| 5

Any help is appreciated!

Regards, Alex

Edit: I had no clue that the database engine was that important. This is running on a MySQL 5 Database (not sure about the complete version string).

Answer 1

There are various ways to do this, depending on the database. Date functions are notoriously database independent.

Here is an approach using a "driver" table with all dates and to use this for a left outer join:

select driver.timestamp, coalesce(t.value, 0) as value
from (select distinct timestamp + n.n as timestamp
      from t cross join
           (select 0 as n union all select 1 union all select 2 union all select 3
           )
     ) driver left outer join
     t;

This version assumes that there are gaps of no more than three days.

In some databases, you can construct the list of dates using a recursive CTE. Such an approach would handle gaps of any size.