[SQL] Comparing sequential rows in a result

[Murray Long]

I'm relatively new to SQL, and am frequently running into the same problem,
How do I compare different rows in a result?

for example:
If I have a table of  events consisting of a time stamp and the event type:

timestamp,     event_type
12:00                 a
12:10                 b
12:20                 a
...

I'd like to be able to select all the 'a' type events and calculate the time
since the previous 'a' event, to get:
timestamp,     event_type,     time_since_last
12:00                 a                             0:20
12:20                 a                             NULL

What's the best way to to accomplish this?


Thanks in advance,
Murray