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