What is a good way to find gaps in a set of datespans?

Question

What is a way to find gaps in a set of date spans?

For example, I have these date spans:

1/ 1/11 - 1/10/11  
1/13/11 - 1/15/11  
1/20/11 - 1/30/11
         


        

Answer 1

Assuming MySQL, something like this would work:

select @olddate := null;

select start_date, end_date, datediff(end_date, @olddate) as diff, @olddate:=enddate
from table
order by start_date asc, end_date asc
having diff > 1;

Basically: cache the previous row's end_date in the @olddate variable, and then do a diff on that "old" value with the currel enddate. THe having clause will return only the records where the difference between two rows is greater than a day.

disclaimer: Haven't tested this, but the basic query construct should work.