Question

I am trying to fabricate an SQL query that will provide these results:

  | Category Title | Subcategory Of |
  -----------------------------------
  |   Category 1   |                |
  |   Category 2   |                |
  |   Category 3   |                |
  |   Category 4   |                |
  |   Category 5   |                |
  |   Category 6   |   Category 4   |
  |   Category 7   |   Category 5   |

This is what my database looks like:

CREATE TABLE `categories` (
  `category_id` int(4) NOT NULL AUTO_INCREMENT,
  `subcategory_id` int(4) NOT NULL,
  `category_title` longtext COLLATE utf8_unicode_ci NOT NULL,
  PRIMARY KEY (`category_id`)
) ENGINE=MyISAM  DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

INSERT INTO `categories` (`category_id`, `subcategory_id`, `category_title`) VALUES
(1, 0, 'Category 1'),
(2, 0, 'Category 2'),
(3, 0, 'Category 3'),
(4, 0, 'Category 4'),
(5, 0, 'Category 5'),
(6, 4, 'Category 6'),
(7, 5, 'Category 7');

I thought that you would use JOIN, but I wasn’t able to mentally think of what kind of query to run, since as far as I knew JOIN was for joining two tables, not two columns. I’m new to these advanced queries (I’m good with INSERT, UPDATE, DELETE, etc. though). Any help is appreciated.

This is what I was trying, which makes no sense really.

SELECT * FROM categories RIGHT JOIN categories ON subcategory_id = category_id

Answer 1

It’s called a self-join. You incldue the table name twice in the query, but giving it two different aliases and then it’s just like a normal join:

SELECT
    C1.category_title AS category_title,
    C2.category_title AS subcategory_of
FROM categories C1
LEFT JOIN categories C2
ON C1.subcategory_id = C2.category_id