Get the count of items related to nested category in sql

mysql, sql

Solution

SELECT parent.title,
( SELECT count(i.id) count FROM items i 
 WHERE category_id IN 
 (
 SELECT child.id FROM categories child WHERE child.lft>=parent.lft AND   
  child.rgt<=parent.rgt AND child.root=parent.root
 )
)
FROM categories parent 
WHERE parent.parent_id=@parent_id;

Please inform me if this does not work

Problem

I have two tables: categories and items. i have stored categories using nested set structure. Categories have items. Items can be only added to leaf nodes of a root category. For eg: Categories ``` Vehicles Bikes Bajaj Automobiles Art & Antiques Amateur Art ``` Items can be added to category Bajaj, Automobiles and Amateur Art in this case. Lets say there are 2 items inside Bajaj, 5 items inside Automobiles, 2 inside Amateur Art For root level categories I want to display as follow: ``` - Vehicles (7 items) - Art & Antiques (2 items) ``` How can I do this ? Here is the sql dump to work with some sample data ``` -- -- Table structure for table `categories` -- CREATE TABLE IF NOT EXISTS `categories` ( `id` int(11) NOT NULL AUTO_INCREMENT, `parent_id` int(11) DEFAULT NULL, `title` varchar(64) COLLATE utf8_unicode_ci NOT NULL, `lft` int(11) NOT NULL, `lvl` int(11) NOT NULL, `rgt` int(11) NOT NULL, `root` int(11) DEFAULT NULL, PRIMARY KEY (`id`), KEY `IDX_3AF34668727ACA70` (`parent_id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci AUTO_INCREMENT=28 ; -- -- Dumping data for table `categories` -- INSERT INTO `categories` (`id`, `parent_id`, `title`, `lft`, `lvl`, `rgt`, `root`) VALUES (22, NULL, 'Vehicles', 1, 0, 8, 22), (23, 22, 'Bikes', 2, 1, 5, 22), (24, 23, 'Bajaj', 3, 2, 4, 22), (25, 22, 'Automobiles', 6, 1, 7, 22), (26, NULL, 'Art & Antiques', 1, 0, 4, 26), (27, 26, 'Amateur Art', 2, 1, 3, 26); -- -------------------------------------------------------- -- -- Table structure for table `items` -- CREATE TABLE IF NOT EXISTS `items` ( `id` int(11) NOT NULL AUTO_INCREMENT, `category_id` int(11) NOT NULL, `title` varchar(100) NOT NULL, PRIMARY KEY (`id`), KEY `FK_403EA91BA33E2D84` (`category_id`) ) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=5 ; -- -- Dumping data for table `items` -- INSERT INTO `items` (`id`, `category_id`, `title`) VALUES (1, 24, 'Pulsor 150 cc'), (2, 24, 'Discover 125 cc'), (3, 27, 'Art of dream'), (4, 25, 'Toyota Car'); -- -- Constraints for dumped tables -- -- -- Constraints for table `categories` -- ALTER TABLE `categories` ADD CONSTRAINT `FK_3AF34668727ACA70` FOREIGN KEY (`parent_id`) REFERENCES `categories` (`id`) ON DELETE SET NULL; -- -- Constraints for table `items` -- ALTER TABLE `items` ADD CONSTRAINT `FK_403EA91BA33E2D84` FOREIGN KEY (`category_id`) REFERENCES `categories` (`id`) ON DELETE CASCADE; ``` root nodes have NULL in the field of parent_id Update: I was able to fetch for roots using this query: ``` SELECT c.id,c.title,cte.co FROM categories c JOIN (SELECT c0_.id,c0_.root,COUNT(i.id) co FROM categories c0_ JOIN items i ON c0_.id=i.category_id WHERE c0_.rgt = 1 + c0_.lft GROUP BY c0_.id ) cte ON cte.root=c.id WHERE c.parent_id is null ``` The above query works for root level category. Now when the user clicks on root level category, I want to do the same. for eg when somebody clicks on vehicles I should get: ``` Bikes (2) Automobiles (5) ``` For that I tried : ``` SELECT c.id,c.title,cte.co FROM categories c JOIN (SELECT c0_.id,c0_.root,COUNT(i.id) co FROM categories c0_ JOIN items i ON c0_.id=i.category_id WHERE c0_.rgt = 1 + c0_.lft GROUP BY c0_.id ) cte ON cte.root=c.id WHERE c.parent_id=1 ``` This returned empty result set. what is wrong in this query ?

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