Apache Casandra is another opensource database management system, it is NoSQL database, which rely on the key value to manipulate and store the data. Casandra is designed to handle very large amount of data. In Casandra there is also provision to store the data across the distributed server which makes the load sharing on all the database server. Distributed data store provides another advantage, which is no single point of failure so data availability increases.
The database is free and open source with a commercial license available (MySQL is now owned by Oracle after they bought Sun). Common applications for MySQL include php and java based web applications that require a DB storage backend, e.g. Dokuwiki, Joomla, xwiki etc
MySQL has one major advantage, since it is free, it is usually available on shared hosting packages and can be easily set up in a Linux, Unix or Windows environment. If a web application requires more than database, requires load balancing or sharding