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I want the following database table:
[Id] [BookType] [Hardcover]
But only when booktype is 1, hardcover may not be null. So whenever booktype is 2 or 3 I should be able to give a null value to hardcover.
Is there anything in my database where I can set this kind of rule?
Your hardcover column must accept null to support type 2 and 3. It is application rule not database rule. You must validate in the application that entity with book type = 1 always has a value set to hardcover.
If you defined inheritance and you have separate type for book of type 1 you can try to make Hardcover property non nullable in the model (but it will most probably don’t work with the code first approach).