如何优化mysql查询获取类别和子类别

Question

I created a script which builds a menu with categories, sub-categories and sub-sub-categories. To get these items I use 3 separate MySQL queries. As follows:

To retrieve the main categories:

SELECT id, name FROM categories WHERE parent_id=0

To retrieve the sub-categories for each category I use:

SELECT id, name FROM categories WHERE parent_id=?

Where the ? is the id of the parent category. I then run the query again for each sub-category.

How can I optimise this script to use less sql queries ? Thank you in advance.

Answer 1

Try this query -

categories+sub categories:

SELECT c1.id, c1.name, c2.id, c2.name FROM categories c1
  LEFT JOIN categories c2
    ON c2.parent_id = c1.id
WHERE c1.parent_id = 0

---

categories+sub categories+sub sub categories:

SELECT c1.id, c1.name, c2.id, c2.name, c3.id, c3.name FROM categories c1
  LEFT JOIN categories c2
    ON c2.parent_id = c1.id
  LEFT JOIN categories c3
    ON c3.parent_id = c2.id
WHERE c1.parent_id = 0

Answer 2

Some database servers have constructs to help these operations. However, MySQL is not one of them.

There are alternative ways to store hierarchies in a database that are more efficient. For a full story, check out this article. It will introduce you to nested sets.

http://mikehillyer.com/articles/managing-hierarchical-data-in-mysql/

Answer 3

An alternative if you only want to the parent category of a category (result row count should be equal to the number of category/subcategory rows) :

SELECT c1.id, c1.name, c2.id, c2.name 
    FROM category c1
       LEFT JOIN category c2 ON c1.parent_id = c2.id