How to create foreign key that is also a primary key in MySQL?

Question

This should be a fairly straightforward question, but I'm unable to find an easy answer. How do you create a foreign key that is also a primary key in MySQL? Here's my current attempt:

CREATE TABLE Sale(
    sale_id CHAR(40),
    PRIMARY KEY(sale_id),
    discount DOUBLE,
    type VARCHAR(255),
    price DOUBLE,
    );

CREATE TABLE Normal_Sale(
    sale_id CHAR(40),
    PRIMARY KEY(sale_id);
);

CREATE TABLE Special_Sale(
    sale_id CHAR(40),
    PRIMARY KEY(sale_id);
);

What am I missing here?

Thanks in advance.

Markup

This design doesn't look right. To me it seems like you could just add a field to Sale which marks it as Normal or Special. This smells – Decrepitate

You are over-normalizing the schema. As Joe Phillips says, a much simpler solution exists. Your design could lead to headaches as the two states should be mutually exclusive (either normal, or special), what prevents a sale to be recorded as both special and normal in your schema? Code, you say? That's it; that's the headache you don't need. – Pastelist

These are just dummy examples. The question is not a design question, it is a technical one. – Markup

Answer 1

Add FOREIGN KEY (sale_id) REFERENCES Sale(sale_id) to each foreign table:

CREATE TABLE Sale(
    sale_id CHAR(40),
    PRIMARY KEY(sale_id),
    discount DOUBLE,
    type VARCHAR(255),
    price DOUBLE
) ENGINE=INNODB;

CREATE TABLE Normal_Sale(
    sale_id CHAR(40),
    PRIMARY KEY(sale_id),
    FOREIGN KEY (sale_id) REFERENCES Sale(sale_id)
) ENGINE=INNODB;

CREATE TABLE Special_Sale(
    sale_id CHAR(40),
    PRIMARY KEY(sale_id),
    FOREIGN KEY (sale_id) REFERENCES Sale(sale_id)
) ENGINE=INNODB;

Just make sure your database is InnoDB which supports Foreign References.

Dufresne

Thank you for seeing this as a technical question. Jut the answer I needed. – Floss