How to 'Unit' Test a database access layer

I have written a .Net application which has many components, some of those components are database access layers which abstract from the rest of the components from where the data comes from.

I have unit tested the rest of the components by mocking the database access layer. One way I have of testing the database access layers is to use create new empty databases on the test servers. This can be slow, and most would argue that it is not a unit tests as I depend on the database server.

What I think I want, is a mocked database which I can use to test my database access layer. The mocked database can be given a schema and process SQL commands as if it were a remote database, but in fact it is all in-memory. Does this exists? Or how else can I test my SQL and database <-> data model code.

To solve my problem you may want to know I am using SQL Server, versions 2008 and later, and my code is written in C#, running with .Net 4.5.

Note: I do not want to use Linq2SQL/EntityFramework to replace my database access layer, as in my experience it results difficult to debug issues and performance problems.