Question

There are two methods which I noticed while designing child tables

Method 1 Example:

Master Table >>Order Table (order_id pk)
Child Table >>order_products 
table (order_id,product_id, quantity,PK (order_id,product_id))

Method 2 example:

Master Table >>Order Table (order_id pk)
Child Table >>order_products table (order_product_id PK, order_id,product_id, quantity)

Question:
Notice that We are using addition order_product_id in the second method. That is my question, is it goo to use combined primary key or introduce a new column like in method 2?

what are pros and cons.
is the anwer depends on the relationship? (in case of one-to-many method 1 is better or in case of many-to-many method 2 is better etc)

Answer 1

the second one’s the choice to go since you have a composite primary key in the first one:

PK (order_id,product_id)

so you always need these two values to refer to a record.

I’d recommend the second one and if you need some restrictions create a unique index on order_id and product_id.