You can use the following command:
mysql -u root -p --one-database destdbname < alldatabases.sql
Where destdbname is your desired database which you want to restore.
Another option which is IMHO much safer, is to extract the DB from an --all-databases dump. Example:
sed -n '/^-- Current Database: `dbname`/,/^-- Current Database: `/p' alldatabases.sql > output.sql
Replace dbname with the desired database name. alldatabases.sql is the name of your sql-dump file. That way you’ll have the seperated DB on file, and then you can restore using a simple mysql command.
(Credits goes to: Darren Mothersele – see his page)