Question

I am stumbling when it comes to how to deal with a database design. I have looked around and found examples like http://www.tomjewett.com/dbdesign/dbdesign.php?page=manymany.php which would work quite well for a shop selling items but I want to build a database that would track sales (or trades) between two users.

Ex John trades cards x1, x2, x3, x4 for Jane’s (4 * y1), y2, and $5.00. (In this example John trades four cards, named x1, x2, x3, x4 for 5 of Jane’s cards, 4 of a card named y1, 1 named y2 and $5.00.)

I would also want Jane to be able counter offer to John for extra cards, or whatever she might want to balance it out in her mind.

Thank you in advance

Answer 1

This is an interesting example of a relational DB design. Essentially, my recommendation would be to create a structure that is as normalized as possible. That means basically that you want to create tables such that you can fill in the blank: “a row in this table represents a unique __.”

For your particular application, I would suggest:

• A table of users, 1 row per user
• A table of cards, 1 row per each card (regardless of how many instances of that card exist in the world – an example would be a row for a 1984 Michael Jordan rookie card by Topps)
• A table of trades, 1 row per trade. You would want the trade row to contain columns for the user1 and user2 involved in the trade, using their IDs from the users table. If you want the ability to counteroffer, I would suggest that a counteroffer is just another row in this table, with a column “original_offer” that references the offer on which a counter-offer is based.
• A table of trade items. Here, you would have 1 row for each item involved in a trade, with the quantity. Each row would have a tradeID that maps to the trades table, a cardID that maps to the cards table, a quantity column, and “from” and “to” columns, each of which map to the userIDs involved.