PHP/SQL/Mysql/phpmyadmin - Foreign Keys - php

Question

I'm building a shop for school purposes. I have finished my shop but i didn't joined tables and used foreign keys (requirement) because i forgot..
I have a newbie 'question' about foreign keys.
PRODUCTS RATING TABLE:
rate_id (id of the rate),
rate, (stars 0 to 5)
comment (user input textarea),
user_id (id of the user who commented the product),
product_id (id of the product that was commented)
USERS TABLE:
`id` (id of the user),
`email` (email of the user),
`name` (name of the user),
`age`,
`username`,
`password`,
`profile_pic`,
`role` (ADMIN/USER)´
When i enter a comment on X product (if (isset($_POST['comment_rating])):
$sql = mysqli_query($link, "SELECT id FROM users WHERE username='$user_id'");
and then later:
$sql = "INSERT INTO products_rating (rate, comment, user_id, product_id)
VALUES ('$rate_text', '$comment', '$user_id', '$product_id');
FINAL: What i want to know is, what do i need to change in my code if i add a foreign key? And should i add a foreign key in this particular case? I must interligate all tables, and I don't understand much about joins / foreign keys. If i wasn't clear let me know. Thank you for you help!

Answer 1

The idea of a foreign key is pretty straightforward.
In the products_rating table, we are storing a value for user_id.
And the value that we stored there is a value of id column from a row in the user table. This establishes a relationship between the row in products_rating an a row in user.
We could get the name and email address of the user that left a product rating
SELECT u.name
, u.email
, r.rate
, r.comment
FROM product_rating r
JOIN user u
ON u.id = r.user_id
WHERE ...
A FOREIGN KEY is a constraint that implements a rule. The rule we want to implement is that we want to allow only valid user.id values to be stored in product_rating.user_id.
For example, if we attempt to store a value of '42' in the product_rating.user_id column, the database is going to check that the value '42' appears in a row in the user table (in the id) column.
The syntax that we use to implement that constraint would be on the product_rating table:
ALTER TABLE `product_rating`
ADD CONSTRAINT -- we are adding a constraint
`FK_product_rating_user` -- the name we assign to the constraint
FOREIGN KEY -- the type of constraint
(`user_id`) -- the column(s) in this table
REFERENCES -- refer to
`user` -- the name of the "foreign" table
(`id`) -- the column(s) in the "foreign" table
We can add some additional configuration that affects behavior...
ON DELETE RESTRICT
ON UPDATE CASCADE
This implements a rule... if we attempt to assign a value to user_id column in this table, and that value is not found in a row in user table id column, an error is thrown, and the statement fails.
This rule also says that if we attempt to remove a row from user, if there are any rows in product_rating that have a user_id value that matches the id value of the row we're deleting, an error is thrown, and the statement fails.
The rule also says that we update a row in user, and assign a new value to the id column, than any rows in product_rating that have a user_id value that matches the old id value will be updated, to assign the new id value to the user_id column... preserving the relationship between the rows.
Also note that this implies a "one-to-many" relationship. A user can be related to zero, one or more product_rating. A product_rating is associated with one user. (If the user_id column allows for NULL values to be stored, a NULL value represents that the row in product_rating is not associated with any user.)
That's just a rough overview of FOREIGN KEY constraints.
The basic idea is that we establish a relationship between rows by storing a common value. In the example we use, a common value of '42' in the id column of a row in the user table, and in the user_id column of one (or more) row(s) in the product rating table.
With no foreign key constraint defined, the database allows us to store any value in the user_id column. We could store '43' or '8670', and it doesn't matter if those values appear in user.id or not.
The foreign key we defined constrains (restricts) what values we can store in the user_id column. When we attempt to add or modify a row in product_rating, the database checks the value of the user_id column, and if a non-NULL value doesn't reference a row in user, an error is raised.
Note that the foreign key constraint does not cause the user_id column to be automatically populated. If we want to establish a relationship to a row in user, we have to supply a value for user_id.