operating on data from multiple rows?

Here is a problem I've run into with an old IMHO poorly designed database:

There is a table ("log") that has fields

username, sessionid, loggedtime, loggeddate, accntstatus

A SELECT might return this data, for example:

bob 1035208 2002-10-11 11:32:00 Start
bob 1035208 2002-10-11 11:38:00 Stop
bob 1052072 2002-10-12 10:05:00 Start
bob 1052072 2002-10-12 10:15:00 Stop

I'm trying to get my head around a SELECT that will return
only one entry per sessionid with a duration instead of two entries for
each. If I had two separate tables for Start and Stop it would
be trivial with a join, but all I can think of is doing a
"SELECT ... WHERE accntstatus = 'Start'" and then grabbing the
sessionid and doing a separate SELECT for every record (and then the
math to get the duration). This seems like a bad idea since thousands
of records are retrived at a time.
Am I missing a better way?

--
Joshua Daniel Franklin
Network Administrator
IOCC.COM