Question

I am in the process of designing the databases for my system. There are a lot of foreign key constraints.

I was wondering whether I could get some advice, whether I should do which of the following:

1) Specify the constraints during table creation itself ie,

CREATE TABLE IF NOT EXISTS abc
    (
    keyword VARCHAR(20) NOT NULL,
    id INT UNSIGNED NOT NULL,
    FOREIGN KEY (id) REFERENCES xyz(id) ON DELETE CASCADE ON UPDATE CASCADE
    )ENGINE=InnoDB;

2)create the table without FK constraints and ‘alter’ the table later on ie,

CREATE TABLE IF NOT EXISTS abc
    (
    keyword VARCHAR(20) NOT NULL,
    id INT UNSIGNED NOT NULL,
    )ENGINE=InnoDB;
ALTER TABLE abc ADD CONSTRAINT fk_constraint FOREIGN KEY (id) REFERENCES xyz(id)
ON DELETE CASCADE ON UPDATE CASCADE;

Table xyz is simply another table with ‘id’ as a primary key.

Answer 1

You may create the FK at once. But this is not always possible because they can refer to each other in a circular fashion. Also, you may want to add columns later, with a FK.

It may be slightly faster to add it at once, because MySQL has to validate and rebuild the table structure for some changes (although I’m not sure adding FKs is one of those). But this process will be reasonably fast on empty tables, so it doesn’t matter much when you add the FK.