constraints.txt

1. No two items can share the same User ID.
Appended PRIMARY KEY to UserID in table Users.

2. All sellers and bidders must already exists as users.
Appended FOREIGN KEY(SellerID) REFERENCES Users to table Items.
Appended FOREIGN KEY(UserID) REFERENCES Users to able Bids.

3. No two items can share the same Item ID.
Appended PRIMARY KEY to ItemID in table Items.

4. Every bid must correspond to an actual item.
Appended FOREIGN KEY(ItemID) REFERENCES Items to table Bids.

5. The items for a given category must all exist.
Appended FOREIGN KEY(ItemID) REFERENCE Items to table CategoryOf.

6. An item cannot belong to a particular category more than once.
Appended PRIMARY KEY(ItemID, Category) to table CategoryOf.

7. The end time for an auction must always be after its start time.
Appended CHECK(Ends>Started) to attribute Ends in table Items.

8. The [Current Price] of an item must always match the Amount of the most recent bid for that item.
*trigger1

9. A user may not bid on an item he or she is also selling.
*trigger2

10. No auction may have two bids at the exact same time.
Appended UNIQUE(ItemID, Time) to table Bids.

11. No auction may have a bid before its start time or after its end time.
*trigger3

12. No user can make a bid of the same amount to the same item more than once.
Appended UNIQUE(UserID, Amount) to table Bids.

13. In every auction, the Number of Bids attribute corresponds to the actual number of bids for that particular item.
*trigger4

14. Any new bid for a particular item must have a higher amount than any of the previous bids for that particular item.
*trigger5

15. All new bids must be placed at the time which matches the current time of your AuctionBase system.
*trigger6

16. The current time of your AuctionBase system can only advance forward in time, not backward in time.
*trigger7

Note: Key constraint, referential integrity constraints and check constraints are implementated in create.sql 
All the triggers are in the triggers folder.