Run this command (if the mysql executable isn’t in your PATH, first go to the directory where the MySQL binary is installed, something like \mysql\bin):
mysql -u username -ppassword databasename < file.sql
(Note that there is no space between -p and the password)
Or if the file is gzipped (like my backups mostly are) then something like this:
gunzip file.sql.gz | mysql -u username -ppassword databasename
or on some systems it might be necessary to add the -c flag to gunzip, like so (to force it to output to stdout):
gunzip -c file.sql.gz | mysql -u username -ppassword databasename