Question

I need some help at understanding how foreign keys and cascades work. I understood the theory but I’m having troubles to apply these to a real world example.

Let’s assume I’ve got the following tables (and an arbitrary number of other tables that may reference table tags):

CREATE TABLE tags (
    id INT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
    name VARCHAR(255) UNIQUE
) Engine=InnoDB;

CREATE TABLE news (
    id INT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
    title VARCHAR(63),
    content TEXT,
    INDEX (title)
) Engine=InnoDB;

So I create a further table to provide the many-to-many relation between news and tags:

CREATE TABLE news_tags (
    news_id INT UNSIGNED,
    tags_id INT UNSIGNED,
    FOREIGN KEY (news_id) REFERENCES news (id) ON DELETE ...,
    FOREIGN KEY (tags_id) REFERENCES tags (id) ON DELETE ...
) Engine=InnoDB;

My requirements to the cascades:

• If I delete a news, all corresponding entries in news_tags should be removed as well.
• Same applies for table x that may be added later with x_tags-table.
• If I delete a tag, all corresponding entries in news_tags and in every further table x_tags should be removed as well.

I’m afraid that I may have to revisit my table structure for this purpose, but that’s alright since I’m only trying to figure out how stuff works.

Any links to good tutorials, SQL-queries or JPA-examples appreciated!

Answer 1

You seem to be proposing something like this, which sounds reasonable to me:

CREATE TABLE tags 
(
 id INTEGER NOT NULL, 
 name VARCHAR(20) NOT NULL, 
 UNIQUE (id), 
 UNIQUE (name)
);

CREATE TABLE news 
(
 id INTEGER NOT NULL, 
 title VARCHAR(30) NOT NULL,
 content VARCHAR(200) NOT NULL,
 UNIQUE (id)
);

CREATE TABLE news_tags
(
 news_id INTEGER NOT NULL, 
 tags_id INTEGER NOT NULL, 
 UNIQUE (tags_id, news_id), 
 FOREIGN KEY (news_id) 
    REFERENCES news (id)
    ON DELETE CASCADE
    ON UPDATE CASCADE, 
 FOREIGN KEY (tags_id) 
    REFERENCES tags (id)
    ON DELETE CASCADE
    ON UPDATE CASCADE
);