Database attach method to upgrade to a new farm

ADDCONTENTDB

stsadm -o addcontentdb -url  http://intranet  -databasename wss_content_hr -databaseserver yoursqlservername\instancename

if you have multiple subsites then you can very well include all of those stsadm database attach command in a batch file and execute it at one stretch.

ex:

save the below in a execute.bat file:

'----
stsadm -o addcontentdb -url http://intranet -databasename wss_content_hr -databaseserver yoursqlservername\instancename >> result.txt

stsadm -o addcontentdb -url  http://intranet/sites/hr -databasename wss_content_hr -databaseserver yoursqlservername\instancename >> result.txt

stsadm -o addcontentdb -url  http://intranet/sites/it -databasename wss_content_hr -databaseserver yoursqlservername\instancename >> result.txt

'----

the above file execute.bat once executed will append result.txt with the upgrade status.

more reference