how to design a schema where the columns of a table are not fixed

Question

I am trying to design a schema where the columns of a table are not fixed. Ex: I have an Employee table where the columns of the table are not fixed and vary (attributes of

Answer 1

Combine your ExtensionColumn tables into one

Property:
    EmployeeID foreign key
    PropertyName string
    PropertyValue string

If you use a monotonic sequence for assigning primary keys in all your object tables then a single property table can hold properties for all objects.