Question

I want to know how to design invoice table.

An invoice include many orders from order table.

Is this invoices table is correctly designed?

order

• order_id (PK)
• User_id (FK)
• total_cost
• order_date
• status

order_item

• order_item_id (PK)
• order_id (FK)
• item_name
• cost

invoice table

• invoice_id (PK)
• invoice_no
• invoice_date
• period_start_date
• period_end_date
• status

invoice_order (an invoice with many orders)

• invoice_order_id (PK)
• invoice_id (FK)
• order_id (FK)

Is invoice_order table necessarry?

I could add invoice_id (FK) field in the order_table instead. The “order. invoice_id” would be updated when I have added a row in the invoice table.

Answer 1

You need the linking table. An order can be in mulitple invoices (if they didn’t pay it!) and an invoice can contain many orders. In a linking table though I would not bother with •invoice_order_id (PK). The PK is a combination of the two FK fields. That guarantees uniqueness and since you are unlikely to have a child table of the link table, you really gain nothing by adding a surrogate key to it. Even if you did the performance difference between joining on two indexed int fields vice one would probably be negligible.