MySQL: Version control get newest revisions - mysql

Question

I have implemented a version control with mysql (and php).
It looks like the following:
categories
-------------
| id | name |
|----|-------
| 1 | a
| 2 | b
| 3 | c
| 4 | d
-------------
revisions
----------------------
| id | cid | current |
|----|-----|----------
| 1 | 1 | 1 |
| 2 | 1 | NULL |
| 3 | 2 | NULL |
| 4 | 3 | 1 |
| 5 | 4 | NULL |
| 6 | 4 | 1 |
----------------------
Each category has multiple revisions assigned to it. A revision is set, if "current" is set to 1 (only one revision per category can be 1, all others are NULL). What I want now is to get every category, which has new reivisions (note, once a revision is submitted it is not set to current immedeately. this is done by a e.g. a moderator). My problem is now getting also each category, which only has one revision with current IS NULL
Therefore the expected result would be:
-------------
| category.id
|------------
| 1
| 2
-------------
all the best
edit://
My current solution is:
SELECT categories.* FROM categories
JOIN revisions as t1 ON
(
t1.cid = categories.id
t1.current IS NULL
)
JOIN revisions as t2 ON
(
t2.id != t1.id
t2.current IS NOT NULL
t2.cid = categories.id
)
WHERE t1.id > t2.id

Answer 1

i made an SQL Fiddle Demo with a different table structure that will fit your needs
CREATE TABLE `categories` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(1) NOT NULL DEFAULT '',
`revision_id` int(11) unsigned DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
INSERT INTO categories
(`id`, `name`, `revision_id`)
VALUES
(1, 'a', 1),
(2, 'b', null),
(3, 'c', 4),
(4, 'd', 6),
(5, 'e', 7),
(6, 'f', 9)
;
CREATE TABLE `revisions` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`category_id` int(11) unsigned NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
INSERT INTO revisions
(`id`, `category_id`)
VALUES
(1, 1),
(2, 1),
(3, 2),
(4, 3),
(5, 4),
(6, 4),
(7, 5),
(8, 6),
(9, 6)
;
Foreign Key Relations
ALTER TABLE `categories`
ADD CONSTRAINT `fk_cat_rev`
FOREIGN KEY (`revision_id`)
REFERENCES `revisions` (`id`) ON UPDATE CASCADE ON DELETE SET NULL;
ALTER TABLE `revisions`
ADD CONSTRAINT `fk_rev_cat`
FOREIGN KEY (`category_id`)
REFERENCES `categories` (`id`) ON UPDATE CASCADE ON DELETE CASCADE;
Check Relations on DELETE
DELETE FROM revisions WHERE id = 7;
DELETE FROM categories WHERE id = 6;
SELECT
cid
FROM (
SELECT
c.id cid,
MAX( r.id ) mrid,
c.revision_id rid
FROM
categories c
JOIN
revisions r ON r.category_id = c.id
GROUP BY
c.id ) tmp
WHERE
COALESCE( mrid, 0 ) <> COALESCE( rid, 0 );
SQL Fiddle DEMO