餐厅食品可选折扣的数据库设计

Question

I have to design a database for a restaurant chain.
The situation is as follows - The customer gets either Item A free OR Item B free (not both), when he purchases Item C.

Following are the two tables in my DB.

For the above example, I will insert an entry for Item C in the HappyHour_Menu_Trans that will generate a new HappyHourMenuTransId. Using this HappyHourMenuTransId, I will insert two entries in the HappyHour_Menu_Subsequent Table.

Now what about the optional part?
I can think of adding a column 'Condition' in which I will put value 'OR' for both entries. This OR will tell me that I can choose only one option.
If I put value 'AND', it will tell me that I will get discounts on both the items.

This solution is fine for me till now, but my senior wants me to avoid using string. I can use a checkbox (that's not the problem!).

The Tricky parts starts from here -

Lets suppose the discount is like this - Buy Item A, and you will get either both (Item B + Item C) free, OR only (Item D + Item E) free.

Now according to my current system I will insert four entries; but how will I know the exact logic from the tables(which two items make a pair?).

Answer 1

I need clarification on the following statement you made.

"Buy Item A, and you will get either both (Item B + Item C) free, OR only (Item D + Item E) free."

Is it only Item D or Item E?

Initially I was thinking this could be solved with some type of referential integrity.

A discount table has the following entries.

(Purchase Item, Free Item)
(A, B)
(A, C)

Create a foreign key relationship between the [HappyHour_Menu_Subsequent] table and these two columns. However, this assumes a 1-to-1 relationship. If you start bundling items, 1-to-1 or 1-to-x, this solution will fail.

Given the fact that the business logic might change, there are two solutions that come to mind.

1 - I would use an INSERT or UPDATE trigger to enforce your business logic if the application does direct inserts and updates to the tables.

CREATE TRIGGER [TRG_ENFORCE_MY_HAPPY_HOUR_DEALS] 
    ON [HappyHour_Menu_Subsequent]
FOR INSERT, UPDATE
AS
BEGIN

-- put your logic here
IF (Violation) 
    RAISERROR();

END

The Violation is just a place holder. You will have to look at the inserted and deleted tables to retrieve the purchased item and free item. Make sure the combination meets your business rules.

Have the front end handle the error if the logic is violated.

2 - If you use stored procedures to perform INSERTS or UPDATES, add the business logic in that code.

Answer 2

In my opinion, create another table called Group_Item, this table will hold the item as a pair, for your example we will have two rows, one row contains Item (B + C) and one row contains Item ( D + E). If implement like this, you can enhance the item length or reduce in pair, for example (B + C + D) or (C + F).

In case if not a pair just a single item like you said above:

The customer gets either Item A free OR Item B free

We will have two rows, one row contains Item A only and another row contains Item B only