How to make COUNT not count(fieldname) NULL values?

Question

I'm setting up requests for e-commerce backend. I have 3 tables: product (with id as index), category (with id as index) and product_category, which binds first 2 tables, since one product can be in several categories and there can be several products in one category.

The request is to get a list of all categories containing names of categories and number of products in this category including zero values (when category contains no products). Last 2 columns of the results shown.

Unfortunatelly, COUNT(fieldname) gives me 1 instead of 0.

Here's my SQL request:

  SELECT product_category.id_category AS pr_cat_cat_id,
         id_product                   AS pr_cat_pr_id,
         product.name                 AS productname,
         categories.id,
         categories.name,
         COUNT ('pr_cat_cat_id')      AS quantity
    FROM product
         LEFT JOIN product_category ON product_category.id_product = product.id
         RIGHT JOIN (SELECT * FROM category) AS categories
             ON categories.id = product_category.id_category
GROUP BY name
ORDER BY id ASC

and get this result:

pr_cat_cat_id   pr_cat_pr_id    productname      id     name        quantity    
1               1               Product "Name1"  1      Category 1  2   
2               3               Product "Name 3" 2      Category 2  2   
NULL            NULL            NULL             3      Category 3  1   
NULL            NULL            NULL             4      Category 4  1   
NULL            NULL            NULL             5      Category 5  1   
NULL            NULL            NULL             6      Category 6  1   

I do expect quantity to be zero on categories without products.

Answer 1

You are counting a constant string value, which is never NULL . Use quotes correctly. You don't need them here:

COUNT(product_category.id_category) AS quantity

You cannot use an alias for the COUNT() . You need to refer to the original column.

Note that your query is malformed. The only things in the select should be name and the aggregation functions.

Answer 2

Column aliases from the same select list can't be referenced. Instead use its original column name:

COUNT(product_category.id_category)

Note: Single quotes are for string literals, and those are never null.