Just a basic question here but it's been bothering me for a while. Do all tables have to be related to another? I'm trying to design a database so far I have 15 tables, 13 of them have relationships with each other creating a regular snowflake pattern, but I have two other tables that only relate to each other, meaning they don't have any relationship to the any of the other 13 tables. Does this mean I should review my design or is this considered OK? 

Thank you for your help in advance.