Pattern for Multi User ASP.NET with SQL/LINQ

I have a SQL+ASP.NET situation for which I suspect there might exists a design pattern.

I have a site where users can register and then enter their own data. The users can only see their own data and never anything else.

My question is: Is there a way to somehow simplify this process. All SQL commands clearly need to be filtered based on the user ID, and ASP Web Forms doesn't really allow this in a simple way. Is there a way to maybe produce LINQ objects or something similar that can automatically extend every SQL command sent to the SQL Server with a User ID parameter?