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

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
Then I have a start and end date of 1/7/11 and 1/14/11.
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. Or I want to return only the datespans up to the first gap encountered.
If this can be done in SQL server that would be good.
I was thinking to go through each date to find out if it lands in a datespan... if it does not then there's a gap on that day.

Answer 1

Jump to 2nd last code block for: *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.
Jump to last code block for: *I want to return only
the datespans up to the first gap
encountered.*
First of all, here's a virtual table to discuss
create table spans (date1 datetime, date2 datetime);
insert into spans select '20110101', '20110110';
insert into spans select '20110113', '20110115';
insert into spans select '20110120', '20110130';
This is a query that will list, individually, all the dates in the calendar
declare #startdate datetime, #enddate datetime
select #startdate = '20110107', #enddate = '20110114'
select distinct a.date1+v.number
from spans A
inner join master..spt_values v
on v.type='P' and v.number between 0 and datediff(d, a.date1, a.date2)
-- we don't care about spans that don't intersect with our range
where A.date1 <= #enddate
and #startdate <= A.date2
Armed with this query, we can now test to see if there are any gaps, by
counting the days in the calendar against the expected number of days
declare #startdate datetime, #enddate datetime
select #startdate = '20110107', #enddate = '20110114'
select case when count(distinct a.date1+v.number)
= datediff(d,#startdate, #enddate) + 1
then 'No gaps' else 'Gap' end
from spans A
inner join master..spt_values v
on v.type='P' and v.number between 0 and datediff(d, a.date1, a.date2)
-- we don't care about spans that don't intersect with our range
where A.date1 <= #enddate
and #startdate <= A.date2
-- count only those dates within our range
and a.date1 + v.number between #startdate and #enddate
Another way to do this is to just build the calendar from #start
to #end up front and look to see if there is a span with this date
declare #startdate datetime, #enddate datetime
select #startdate = '20110107', #enddate = '20110114'
-- startdate+v.number is a day on the calendar
select #startdate + v.number
from master..spt_values v
where v.type='P' and v.number between 0
and datediff(d, #startdate, #enddate)
-- run the part above this line alone to see the calendar
-- the condition checks for dates that are not in any span (gap)
and not exists (
select *
from spans
where #startdate + v.number between date1 and date2)
The query returns ALL dates that are gaps in the date range #start - #end
A TOP 1 can be added to just see if there are gaps
To return all records that are before the gap, use the query as a
derived table in a larger query
declare #startdate datetime, #enddate datetime
select #startdate = '20110107', #enddate = '20110114'
select *
from spans
where date1 <= #enddate and #startdate <= date2 -- overlaps
and date2 < ( -- before the gap
select top 1 #startdate + v.number
from master..spt_values v
where v.type='P' and v.number between 0
and datediff(d, #startdate, #enddate)
and not exists (
select *
from spans
where #startdate + v.number between date1 and date2)
order by 1 ASC
)

Answer 2

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.

Answer 3

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.