What is SQL Express LocalDB?

As a developer, the chances are you are well familiar with SQL Server and it's many editions. Chances are you have also used SQL Server Compact Edition (CE), or just Compact as it is often called.

SQL Server Compact is a light-weight database with a footprint of only several megabytes (think four). Then, the next-largest edition is the free SQL Server Express edition, which is fully compatible with the commercial, "big" editions of SQL Server like the Standard Edition.

In SQL Server "Denali", the team is working on an "in-between" solution called SQL Server LocalDB, which is smallish in the footprint (around 150 MB), but would still be convenient to use like SQL Server Compact. The LocalDB should support features like stored procedures (SPs), the Geometry and Geography data types, and so on.