Question

I m working on a very weird problem with SQL where I have to compare previous rows

Number    start_date                      end_date    
 -----     -------                      ------------
 1    2011-06-07 00:00:00.000         2011-07-10 00:00:00.000   
 2    2011-10-11 00:00:00.000         2011-10-11 00:00:00.000   
 3    2011-10-26 00:00:00.000         2011-10-29 00:00:00.000   
 4    2011-10-29 00:00:00.000         2011-11-15 00:00:00.000   

Here , I have to compare the start_date and end_date on the two different line and create a view out of it.
(If the start_date is less than the previous end_date , then criteria is set to 1).

Well it should compare 2011-10-26 00:00:00.000 for 3 and 2011-10-27 00:00:00.000 on 2 for 30 days

Number    start_date                      end_date                 Criteria
-----     -----------                   ----------------          ------------
1   2011-06-07 00:00:00.000       2011-07-10 00:00:00.000            0
2   2011-10-11 00:00:00.000       2011-10-11 00:00:00.000            0
3   2011-10-26 00:00:00.000       2011-10-29 00:00:00.000            1
4   2011-10-30 00:00:00.000       2011-11-15 00:00:00.000            1   

I m confused how should I proceed with this.

Any help would be helpful !!!!

Thanks !!!

Answer 1

The most straightforward way to do this is to use a subquery:

select A.number, a.start_date, a.end_date,
    CASE WHEN start_date < dateadd(d,30,(select TOP(1) b.end_date
     from mytable B
     where B.number < A.number
     order by B.number desc)) then 1 else 0 end Criteria
from mytable A

Note: If the start date is the 29th day following the previous row’s end date, Criteria becomes 1. By the 30th day onwards, it is 0. Tweak the 30 in the query as required.

Sample:

create table mytable (
    Number int primary key,
    start_date datetime,
    end_date datetime);

insert mytable
select 1, '2011-06-07', '2011-07-10' union all
select 2, '2011-10-11', '2011-10-27' union all
select 3, '2011-10-26', '2011-10-29' union all
select 4, '2011-10-29', '2011-11-15'

Result:

number  start_date  end_date    Criteria
1   2011-06-07 00:00:00.000 2011-07-10 00:00:00.000 0
2   2011-10-11 00:00:00.000 2011-10-27 00:00:00.000 0
3   2011-10-26 00:00:00.000 2011-10-29 00:00:00.000 1
4   2011-10-29 00:00:00.000 2011-11-15 00:00:00.000 0