Sql Server Express edition is a small version of a sql server 2005 and that provides data storage capability up to 4 gb. It is easily ships with the each application so many users are using this version of sql server instead Microsoft access as a database for windows base application.
One of the problem using sql server instances is whenever you compile it will overwrite the existing database.
For Example, if you perform a insert operations from application and add new row to the database then again after compile the application. It will over right that database with older
I have found a simple solutions to get rid of this problem. In solution browse to the database and select the database (.mdf files) then go to the property window. There is a property called 'Copy to output' select property and select 'Do not copy'.
Now when you compile the application it will not copy your older database to the bin directory.