Dear oneway, that is the key. Such problem is just occured in ERD. I should design a tree structure database like catalogue-catalogue-catalogue-item. and ...
The number of level should be up to 100. There should be both category and item in each level. In my opinion, there should be the same table structure from level2 to level20 ( PK in the level and FK to the upper level). Now, even though I have thought it over when designing, I am still afraid that my boss will change his idea to add one column to all the tables.