Why we're using MySQL

From the manual:
very fast, multi-threaded, multi-user, and robust SQL database server...intended for mission-critical, heavy-load production systems as well as for embedding into mass-deployed software.

Open Source relational database management system that works in client/server or embedded systems on many different platforms

MySQL Server was originally designed to work with medium-sized databases (10-100 million rows, or about 100MB per table) on small computer systems. Today MySQL Server handles terabyte-sized databases, but the code can also be compiled in a reduced version suitable for hand-held and embedded devices.

"We use MySQL Server with databases that contain 50 million records. We also know of users who use MySQL Server with 60,000 tables and about 5,000,000,000 rows."