The database used on the phone is based on SQL Server Compact Edition (SQL CE). SQL CE shares a common API with the other Microsoft SQL Server editions. Each SQL CE database resides in a single .sdf file, which can be up to 4GB in size. The .sdf file can be encrypted, using a password, with 128-bit encryption for data security.
On Windows Phone, the database filename is required to use the .sdf file extension. A
NotSupportedException is raised if you attempt to create a database with a name not ending in .sdf.