Question

I have two things to do: optimize table, write a query.

customer
id    customer    year_in_business   is_discountable   customers_friend_id 
1     a           2                  true              2
2     b           3                  false             1
3     c           2                  true              1
4     d           3                  true              null

Here customer_friend_id is pointing back to pk(id) of the same table.

product 
id   product 
1     xxx
2     yyy
3     aaaa

customer_product
customer_id  product_id
1             2
2             3
3             1

Questions

1. How to make these tables flexible or extensible in the future?
2. Get Name of each customer that has a friend and the name of that customer’s friend.

Answer 1

Here is one suggestion: remove customer.customers_friend_id and create a new CustomerFriend table like so:

CustomerFriend
customerFriendID  customerID  friendID
1                 1           2
2                 2           1
3                 3           1

This will allow customers with more than one friend, and avoids nulls for customers without friends.