Entity-Relationship Diagram Redundancy: store, product, orders, categories

Question

I'm trying design a model which allows a user to be a buyer and seller with a single account, but some teachers told me that this diagram is wrong because it has redundancy.

I had reviewed the diagram but I haven't found a way to solve this redundancy. In the table orders I need to know who is a buyer, so for this reason I didn't delete this from the table. Some ideas?

Answer 1

The only thing that are "redundant" (not normalized to be exact) in your scheme is this :

You don't need to make a special ID, a composite PK is enough.

-------------------
|   ORDERPRODUCT  |
-------------------
| PK | PRODUCT_ID |
| PK | ORDER_ID   |
-------------------

ADD CONSTRAINT pk 
PRIMARY KEY (PRODUCT_ID, ORDER_ID);