How do foreign keys work?

Question

I've mostly used MyISAM tables before, which don't support foreign keys. Looking on stack overflow I didn't find a nice, concise explanation of what a foreign key is actually doing. I'm mostly interested in join tables, where you would have a schema like this:

customers
id category_id

products
id category_id 

categories
id

customerproducts
customer_id product_id

If I have foreign keys on customerproducts, it will ensure that only valid customers and only valid products get into that table, but what about if I try to add a Product from the phones category to a customer earmarked as one only interested in copiers? Will this cause the foreign key constraints to be violated?

Answer 1

I'm mostly interested in join tables, where you would have a schema like this:

You wouldn't have a schema like that--it doesn't represent the facts you're interested in. Let's sketch out some tables in SQL. (Tested in PostgreSQL) First, customers and products.

-- Customer names aren't unique.
create table customers (
  cust_id integer primary key,
  cust_name varchar(15) not null
);
insert into customers values (1, 'Foo'), (2, 'Bar');

-- Product names are unique.
create table products (
  prod_id integer primary key,
  prod_name varchar(15) not null unique
);
insert into products values 
(150, 'Product 1'), (151, 'Product 2'), (152, 'Product 3');

There are different categories for products.

create table categories (
  cat_name varchar(15) primary key
);
insert into categories values ('Cable'), ('Networking'), ('Phones');

Each product might appear in several categories.

create table product_categories (
  prod_id integer not null references products,
  cat_name varchar(15) not null references categories,
  primary key (prod_id, cat_name)
);

insert into product_categories values 
(150, 'Cable'), (150, 'Networking'), (151, 'Networking'), (152, 'Phones');

A customer might be interested in several categories of products.

create table customer_category_interests (
  cust_id integer not null references customers,
  cat_name varchar(15) not null references categories,
  primary key (cust_id, cat_name)
);

-- Nobody's interested in phones
insert into customer_category_interests values
(1, 'Cable'), (1, 'Networking'), (2, 'Networking');

If I have foreign keys on customerproducts, it will ensure that only valid customers and only valid products get into that table, but what about if I try to add a Product from the phones category to a customer earmarked as one only interested in copiers?

Customers aren't interested in every product in their preferred categories. Note the overlapping foreign key constraints.

create table product_interests (
  cust_id integer not null,
  prod_id integer not null,
  cat_name varchar(15) not null,
  foreign key (cust_id, cat_name) references customer_category_interests,
  foreign key (prod_id, cat_name) references product_categories,
  primary key (cust_id, prod_id, cat_name)
);

insert into product_interests values
(1, 150, 'Cable'), (2, 150, 'Networking');

This next insert will fail, because customer 1 isn't interested in phones.

insert into product_interests values
(1, 152, 'Phones');

ERROR:  insert or update on table "product_interests" violates foreign key constraint "product_interests_cust_id_fkey"
DETAIL:  Key (cust_id, cat_name)=(1, Phones) is not present in table "customer_category_interests".