SQL Server CE
Microsoft SQL Server Compact or SQL CE is an embedded relational databases intended for use both on mobile devices and desktops. SQL Compact shares common programming interface with other SQL Server editions, making it easy to use for .NET developers familiar with standard SQL Server.
There are many limitations compared to a standard SQL Server instance, but that’s expected for a small embedded database engine. You can’t write stored procedures, for example - but one of the main purposes of stored procedures is to avoid frequent network round-trips to the server. With embedded databases, stored procedures have less appeal. Also, XML data type is not supports in SQL Compact.
SQL Server Compact holds its databases in a single file with .SDF extension. However, that extension is just a recommendation, so any extension can be used for the database file. SDF files can be compressed and encrypted. If database is created with password specified, it will be encrypted and will never allow access without correct password.
Some of the well-known competitors to SQL Server Compact are SQLite and Pervasive PSQL, which run on many more platforms compared to SQL CE.