Question:
What is the difference between two dates, date1 and date2, when date2 is in the next record?
Answer:
Use this code:
select datediff(laterone.date1, earlierone.date1) from table earlierone, table lateronewhere earlier.id > later.id
The idea here is to use the same table twice, with an alias so that you remember the perspective of the table as to earlier or later, then do the subtraction between them.