If any of you have the error trying to connect to the SQL Server LocalDB using (localdb)\V12.0. and failed, looks like MS had moved away from numbering the automatic instances and started naming it as “MSSQLLocalDB” so instead of using the above what you have to use is below:
Posted by Microsoft on 5/19/2014 at 11:45 PMHi, Thank you for your feedback. In SQL14, we moved away from the numbering/versioning for the automatic instance and named it “MSSQLLocalDB” instead of “v12.0”. The name of the automatic instance in the SQL14 documentation for LocalDB at http://msdn.microsoft.com/en-us/library/hh510202(v=sql.120).aspx has been fixed. If you require an instance to be named “v12.0”, you can create a named instance with that name.
Thanks & Regards, Pooja Harjani, Sr. Program Manager, SQL Server, Microsoft. (Source : MS Connect)