从MySQL中的两个联接表获取多个计数

Question

So I have two tables, categories and designs. I want to construct a query that will fetch all categories, along with the count of any sub categories (categories.parent_id equal to the categories.id) AND the count of any designs (design.category_id equal to categories.id)

If I try to just get one of these counts, everything works fine, but when I try for both with the following code, the count for both is the same number (and not the correct number) for either.

        $this->db->select('categories.id AS id, categories.parent_id AS parent_id, categories.title AS title, 
        categories.description AS description, categories.img_path AS img_path, COUNT(designs.id) AS design_count, 
        COUNT(sub_categories.id) as sub_category_count');
        $this->db->from('categories');
        $this->db->join('designs',                          'categories.id = designs.category_id', 'left');
        $this->db->join('categories as sub_categories',     'categories.id = sub_categories.parent_id', 'left');
        $this->db->group_by('categories.id');

Any help will be much appreciated, cheers!

Answer 1

Assuming that the root categories do not contain designs, here is the query that returns the necessary information:

SELECT category.id, category.title, subcategory.id, designs.id
FROM categories category
LEFT JOIN categories subcategory ON category.id = subcategory.parent_id
LEFT JOIN designs ON subcategory.id = designs.category_id
WHERE category.parent_id IS NULL

Now all you need to do is to apply grouping:

SELECT category.id, category.title, COUNT(DISTINCT subcategory.id), COUNT(designs.id)
FROM categories category
LEFT JOIN categories subcategory ON category.id = subcategory.parent_id
LEFT JOIN designs ON subcategory.id = designs.category_id
WHERE category.parent_id IS NULL
GROUP BY category.id, category.title

The key here is the use of COUNT(DISTINCT ...) .

Answer 2

SELECT c.id,c.title,
       IFNULL(sc.counted,0) AS subcategories,
       IFNULL(d.counted,0) AS designs
FROM categories c 
     LEFT JOIN 
     ( SELECT parent_id,COUNT(*) AS counted 
         FROM categories GROUP BY parent_id ) sc
       ON c.id=sc.parent_id
     LEFT JOIN
     ( SELECT category_id,COUNT(*) AS counted 
         FROM designs GROUP BY category_id ) d
       ON c.id=d.category_id
WHERE c.parent_id IS NULL ;

should get you the desired numbers as raw SQL.