How do foreign keys work? - database

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".