How to select overlapping date ranges in SQL

Question

I have a table with the following columns : sID, start_date and end_date

Some of the values are as follows:

1   1995-07-28  2003-07-20 
1   2003-07-21  2         


        

Answer 1

One way of doing this reasonably efficiently is

WITH T1
     AS (SELECT *,
                MAX(end_date) OVER (PARTITION BY sID ORDER BY start_date) AS max_end_date_so_far
         FROM   YourTable),
     T2
     AS (SELECT *,
                range_start = IIF(start_date <= LAG(max_end_date_so_far) OVER (PARTITION BY sID ORDER BY start_date), 0, 1),
                next_range_start = IIF(LEAD(start_date) OVER (PARTITION BY sID ORDER BY start_date) <= max_end_date_so_far, 0, 1)
         FROM   T1)
SELECT SId,
       start_date,
       end_date
FROM   T2
WHERE  0 IN ( range_start, next_range_start ); 

if you have an index on (sID, start_date) INCLUDE (end_date) this can perform the work with a single ordered scan.