Question

Is there an equivalent to a grouped primary key using an InnoDB MySQL database and allowing auto-increment on the second key?

What I am trying to implement is a table to store image URL’s based on a parent object ID something like:

CREATE TABLE images (
   parent_id INT(11) NOT NULL,
   image_id INT(11) AUTO_INCREMENT, //will this work with a grouped primary key?
   source_url VARCHAR(255) NOT NULL,
   caption VARCHAR(255) NOT NULL,
) ENGINE = InnoDB;

So for MyISAM I could do something like:

PRIMARY KEY(parent_id, image_id)

[edit] Using a trigger would be something like: (sorry my SQL is not very strong)

delimiter //
DROP TRIGGER IF EXISTS test_img_trigger;
CREATE TRIGGER test_img_trigger BEFORE INSERT ON venue_images
FOR EACH ROW
BEGIN
DECLARE img_id INT UNSIGNED DEFAULT 0;

SELECT image_id + 1 INTO img_id FROM venue_images WHERE venue_id = NEW.venue_id;
UPDATE venue_images SET image_id = img_id WHERE venue_id = NEW.venue_id;

END IF;
END;//
delimiter;

Answer 1

In case this can help anyone else this was my solution.

I could not simply use a trigger as MySQL does not yet (or at least my version 5.1.53) support the ability to use a trigger to update the table the trigger is called on. So I created a sequence table:

CREATE TABLE image_seq (
    parent_id INT(11) NOT NULL,
    catagory INT(11) NOT NULL,
    next_seq_id INT(11) NOT NULL,
    KEY(catagory),
    PRIMARY KEY(parent_id, catagory);
);

And then created a trigger on the images table:

CREATE TRIGGER bi_image_trig
BEFORE INSERT ON images
FOR EACH ROW
BEGIN

DECLARE id INT UNSIGNED DEFAULT 1;
 SELECT next_seq_id + 1 INTO id FROM image_seq WHERE parent_id=NEW.parent_id AND       catagory=NEW.catagory;
SET NEW.image_id = id;
UPDATE image_seq SET next_seq_id=id WHERE parent_id=NEW.parent_id AND catagory=NEW.catagory;

END;

Not sure if its the optimal solution but it works and allows me to keep track of image numbers in catagories for each parent object.