Let's add a table of products that we can use to demonstrate aggregate functions with:
CREATE TABLE product( product_id INT AUTO_INCREMENT PRIMARY KEY, name VARCHAR(40), category_id INT NOT NULL ); INSERT INTO product(name, category_id) VALUES('20" TV',3),('36" TV',3), ('Super-LCD 42"',4),('Ultra-Plasma 62"',5),('Value Plasma 38"',5), ('Power-MP3 5gb',7),('Super-Player 1gb',8),('Porta CD',9),('CD To go!',9), ('Family Talk 360',10); SELECT * FROM product; +------------+-------------------+-------------+ | product_id | name | category_id | +------------+-------------------+-------------+ | 1 | 20" TV | 3 | | 2 | 36" TV | 3 | | 3 | Super-LCD 42" | 4 | | 4 | Ultra-Plasma 62" | 5 | | 5 | Value Plasma 38" | 5 | | 6 | Power-MP3 128mb | 7 | | 7 | Super-Shuffle 1gb | 8 | | 8 | Porta CD | 9 | | 9 | CD To go! | 9 | | 10 | Family Talk 360 | 10 | +------------+-------------------+-------------+Now let's produce a query that can retrieve our category tree, along with a product count for each category:
SELECT parent.name, COUNT(product.name) FROM nested_category AS node , nested_category AS parent, product WHERE node.lft BETWEEN parent.lft AND parent.rgt AND node.category_id = product.category_id GROUP BY parent.name ORDER BY node.lft; +----------------------+---------------------+ | name | COUNT(product.name) | +----------------------+---------------------+ | ELECTRONICS | 10 | | TELEVISIONS | 5 | | TUBE | 2 | | LCD | 1 | | PLASMA | 2 | | PORTABLE ELECTRONICS | 5 | | MP3 PLAYERS | 2 | | FLASH | 1 | | CD PLAYERS | 2 | | 2 WAY RADIOS | 1 | +----------------------+---------------------+This is our typical whole tree query with a COUNT and GROUP BY added, along with a reference to the product table and a join between the node and product table in the WHERE clause. As you can see, there is a count for each category and the count of subcategories is reflected in the parent categories.
Adding New NodesNow that we have learned how to query our tree, we should take a look at how to update our tree by adding a new node. Let's look at our nested set diagram again:
If we wanted to add a new node between the TELEVISIONS and PORTABLE ELECTRONICS nodes, the new node would have lft and rgt values of 10 and 11, and all nodes to its right would have their lft and rgt values increased by two. We would then add the new node with the appropriate lft and rgt values. While this can be done with a stored procedure in MySQL 5, I will assume for the moment that most readers are using 4.1, as it is the latest stable version, and I will isolate my queries with a LOCK TABLES statement instead:
LOCK TABLE nested_category WRITE; SELECT @myRight := rgt FROM nested_category WHERE name = 'TELEVISIONS'; UPDATE nested_category SET rgt = rgt + 2 WHERE rgt > @myRight; UPDATE nested_category SET lft = lft + 2 WHERE lft > @myRight; INSERT INTO nested_category(name, lft, rgt) VALUES('GAME CONSOLES', @myRight + 1, @myRight + 2); UNLOCK TABLES; We can then check our nesting with our indented tree query: SELECT CONCAT( REPEAT( ' ', (COUNT(parent.name) - 1) ), node.name) AS name FROM nested_category AS node, nested_category AS parent WHERE node.lft BETWEEN parent.lft AND parent.rgt GROUP BY node.name ORDER BY node.lft; +-----------------------+ | name | +-----------------------+ | ELECTRONICS | | TELEVISIONS | | TUBE | | LCD | | PLASMA | | GAME CONSOLES | | PORTABLE ELECTRONICS | | MP3 PLAYERS | | FLASH | | CD PLAYERS | | 2 WAY RADIOS | +-----------------------+If we instead want to add a node as a child of a node that has no existing children, we need to modify our procedure slightly. Let's add a new FRS node below the 2 WAY RADIOS node:
LOCK TABLE nested_category WRITE; SELECT @myLeft := lft FROM nested_category WHERE name = '2 WAY RADIOS'; UPDATE nested_category SET rgt = rgt + 2 WHERE rgt > @myLeft; UPDATE nested_category SET lft = lft + 2 WHERE lft > @myLeft; INSERT INTO nested_category(name, lft, rgt) VALUES('FRS', @myLeft + 1, @myLeft + 2); UNLOCK TABLES;In this example we expand everything to the right of the left-hand number of our proud new parent node, then place the node to the right of the left-hand value. As you can see, our new node is now properly nested:
SELECT CONCAT( REPEAT( ' ', (COUNT(parent.name) - 1) ), node.name) AS name FROM nested_category AS node, nested_category AS parent WHERE node.lft BETWEEN parent.lft AND parent.rgt GROUP BY node.name ORDER BY node.lft; +-----------------------+ | name | +-----------------------+ | ELECTRONICS | | TELEVISIONS | | TUBE | | LCD | | PLASMA | | GAME CONSOLES | | PORTABLE ELECTRONICS | | MP3 PLAYERS | | FLASH | | CD PLAYERS | | 2 WAY RADIOS | | FRS | +-----------------------+ Deleting Nodes