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

I want to be able to tell that between 1/10/11 and 1/13/11 there is a gap so the start and end date is not possible.

I think you're asking this question: does the data in your table have a gap between the start date and the end date?

I created a one-column table, date_span, and inserted your date spans into it.

You can identify a gap by counting the number of days between start date and end date, and comparing that the the number of rows in date_span for the same range.

select 
  date '2011-01-14' - date '2011-01-07' + 1 as elapsed_days,  
  count(*) from date_span 
where cal_date between '2011-01-07' and '2011-01-14';

returns

elapsed_days count    
--           --
8            6

Since they're not equal, there's a gap in the table "date_span" between 2011-01-07 and 2011-01-14. I'll stop there for now, because I'm really not certain what you're trying to do.