I made a mess of it. :'-( I explained it better in the post with id 10. I want to use a table to define topics and subtopics. In the records I will use a (NOT NULL) field topicID that points to the correct main topic. I want to use a link table to link the subtopics to a record. But it should not be possible to link the topic as a subtopic. Can this be done in an elegant way? When I have in topic table: 1 Work 2 Familie 3 Friend 4 Hobbies When a record with id 17 has as topicID 1 then in the link table the following would be acceptable: 17 2 17 4 But what should not be acceptable is: 17 1 How can this best be done?