Using references in MYSQL

Question

I got an answer to another question here:

DB Schema For Chats?

It's a great answer, but I am not understanding the bit about references. I can do SQL statements but I have never used references.

1. What are they used for?
2. How are they used?
3. Give an example please

Answer 1

The REFERENCES keyword is part of a foreign key constraint and it causes MySQL to require that the value(s) in the specified column(s) of the referencing table are also present in the specified column(s) of the referenced table.

This prevents foreign keys from referencing ids that do not exist or were deleted, and it can optionally prevent you from deleting rows whilst they are still referenced.

A specific example is if every employee must belong to a department then you can add a foreign key constraint from employee.departmentid referencing department.id.

Run the following code to create two test tables tablea and tableb where the column a_id in tableb references the primary key of tablea. tablea is populated with a few rows.

CREATE TABLE tablea (
    id INT PRIMARY KEY,
    foo VARCHAR(100) NOT NULL
) Engine = InnoDB;

INSERT INTO tablea (id, foo) VALUES
(1, 'foo1'),
(2, 'foo2'),
(3, 'foo3');

CREATE TABLE tableb (
    id INT PRIMARY KEY,
    a_id INT NOT NULL,
    bar VARCHAR(100) NOT NULL,
    FOREIGN KEY fk_b_a_id (a_id) REFERENCES tablea (id)
) Engine = InnoDB;

Now try these commands:

INSERT INTO tableb (id, a_id, bar) VALUES (1, 2, 'bar1');
-- succeeds because there is a row in tablea with id 2

INSERT INTO tableb (id, a_id, bar) VALUES (2, 4, 'bar2');
-- fails  because there is not a row in tablea with id 4

DELETE FROM tablea WHERE id = 1;
-- succeeds because there is no row in tableb which references this row

DELETE FROM tablea WHERE id = 2;
-- fails because there is a row in tableb which references this row

Important note: Both tables must be InnoDB tables or the constraint is ignored.