Microsoft has rather quietly introduced a new version of SQL Express, created specifically for developers.
The new version, SQL Express LocalDB, provides the same T-SQL, programming surface and client-side providers as the regular SQL Server Express, but is designed to be easier to manage. This means that if you want to write an app that runs against SQL Server you no longer have the hassle of installing and managing a full instance of SQL Server Express. It is also intended to work well as an embedded database.
The thinking behind this move, according to the SQLExpress blog, is that SQL Express is being used for two distinct needs. On one hand it is a free edition of SQL Server, where would-be users of the full version can learn how to install, manage and program in a way that is 100% compatible with other editions.
In the case of developers writing applications targeting SQL Server, though, the programming aspects need to be identical, but you don't really want the extras to make it compatible in terms of management and administration. Because of this, in SQL Server "Denali" Microsoft decided to change the approach it and introduce a dedicated version of SQL Express for developers - LocalDB - that delivers the simplicity and yet is compatible with other editions of SQL Server at the API level.
It uses the same sqlservr.exe as other editions of SQL Server, you use the same client-side providers (ADO.NET, ODBC, PDO etc) to connect to it, and you can operate on its data using T-SQL. It doesn't create any database services; LocalDB processes are started and stopped automatically when needed. When an application tries to connect to LocalDB, the provider checks whether the LocalDB instance for the current user is started. If it's already started the application will connect to it, if not the LocalDB instance for the current user will be started and then the provider proceeds to connect to it.
The blog makes clear that LocalDB is not a replacement for SQL Server Express -- it is an addition to SQL Server Express lineup. While LocalDB is meant for developers, the regular SQL Server Express will continue to exist as a free SQL Server edition.
Introducing LocalDB, an improved SQL Express