To generate the backup of mysql database using cron job refer following three format as per your requirement.

1) Cron job to generate  database backup in  .sql format

* * * * * /usr/bin/mysqldump -u databaseusername databasename -ppassword > /path of database database.sql

2) Cronjob to compress database backup in .bz2 format

* * * * * /usr/bin/mysqldump -u datbaseusername databasename -pdatbasepassword|bzip2 -c > /path of database database.bz2

3)  Cronjob to compress database backup in .gz format

* * * * * /usr/bin/mysqldump -u databaseusername databasename -ppassword|gzip > /path of databasedatabase.gz