FIND_IN_SET universal symbol?

Question

I would like to create query like this:

$get_products = "SELECT P.*, C.`category_name`, GROUP_CONCAT(`category_name` SEPARATOR ', ') AS `cat` 
                   FROM `products` P 
                   NATURAL JOIN 
                     `categories` C 
                   NATURAL JOIN 
                     `product_to_categories`";
                   WHERE 
                     FIND_IN_SET(`category_name`, :selected_categories) 
                     AND `product_price` BETWEEN :price_min AND :price_max 
                  GROUP BY `product_name` 
                  ORDER BY 1 DESC ";
               
$db = getConnection();
$stmt = $db->prepare($get_products);
$stmt -> bindValue(selected_categories, $selected_categories);
$stmt -> bindValue(price_min, $price_min);
$stmt -> bindValue(price_max, $price_max);

the problem is that string $selected_categories in some cases is empty, and when it is empty obviously nothing is found.
Is there any parameter which I can use (for example $selected_categories = '*'; ) and it will cause that FIND_IN_SET will find everything? or should I create completely new query for case when $selected_categories is empty?

Answer 1

You can solve this with simple logic

WHERE 
(
   :selected_categories = '*' OR
   FIND_IN_SET(`category_name`, :selected_categories) 
)
AND `product_price` BETWEEN :price_min AND :price_max 

Answer 2

You can use a variable to reference the same replacement parameter twice:

 WHERE IF((@selected_categories:=:selected_categories)='*',1,FIND_IN_SET(category_name,@selected_categories))

and test some value you want to mean all ('*' in this example, you could use ='' or IS NULL or something else).

Some prefer using the SQL standard CASE instead of IF; that would look like:

WHERE
    CASE
        WHEN (@selected_categories := :selected_categories) = '*' THEN 1
        ELSE FIND_IN_SET(category_name, @selected_categories)
    END