Question

I have a table which lists every date within the year. Each row also contains a period number from 1 to 13 which loop around each year.
It looks something like this:

|        Date         | Period |
| 2012-12-27 00:00:00 |   12   |
| 2012-12-28 00:00:00 |   12   |
| 2012-12-29 00:00:00 |   13   |
| 2012-12-30 00:00:00 |   13   |
| 2012-12-31 00:00:00 |   13   |
| 2013-01-01 00:00:00 |   13   |
| 2013-01-02 00:00:00 |   13   |
|        . . .        |   13   | 
| 2013-02-10 00:00:00 |   01   |
|        . . .        |   01   | 
| 2013-03-14 00:00:00 |   02   |
|        . . .        |   02   | 
| 2013-05-05 00:00:00 |   03   |

As you can see above, period 13 starts on the 29th of dec 2012 and finishes on the 09th of Febuary 2013.
I work this out by getting the first and last dates for period 13.

I need to write a query that will get the period start and end dates for the current period.
But I’m having problems because the periods sometimes occur twice a year and sometimes overlap between the years like this example does.

A few more examples:

If getDate() = ‘2013-02-25 13:45:00’ then the period should be 01 and the start and end dates would be ‘2013-02-10 00:00:00’ and ‘2013-03-14 00:00:00’

If getDate() = ‘2013-03-15 00:00:00’ then the period should be 02 and the start and end dates would be ‘2013-03-14 00:00:00’ and ‘2013-05-05 00:00:00’

I really hope this makes sense. Let me know if I need to clarify it! Thanks for your help 🙂

Answer 1

If you were using SQL Server 2012, you would use lag or lead for this. Instead, I’m going to use a correlated subquery:

select min(period) as period, MIN(date), MAX(date)
from (select t.*,
             (select min(date) from t t2 where t2.period <> t.period and t2.date > t.date) as nextp
      from t
     ) t
group by nextp

The inner subquery gets the date of the next period. This is the same for consecutive records with the same period record. I can then use this for grouping.