calculated field returns NULL instead of the expected value

I am trying to calculate a percentage as a derived column in my query.
My query is returning all the expected data but NULL on the percentage column.

SELECT items_type_catalog.id,
       items_type_catalog.description, 
       items_header.tipo, items_type_catalog.color, 
       @solved :=COUNT(CASE WHEN (items_header.stato = 4 OR items_header.stato= 5) THEN 1 END) AS solved,            
       @tutti:=count(items_header.id) as tutti, 
       (ROUND(@solved/@tutti,0)*100) as percentage 
FROM items_type_catalog LEFT JOIN items_header ON items_type_catalog.id=items_header.tipo AND items_header.progetto=1 
GROUP BY items_type_catalog.id, tipo

I cannot see what I am doing wrong on this point. Why is it not doing the math and returns NULL?

>Solution :

https://dev.mysql.com/doc/refman/8.0/en/user-variables.html

The order of evaluation for expressions involving user variables is undefined. For example, there is no guarantee that SELECT @a, @a:=@a+1 evaluates @a first and then performs the assignment.

This means that the usage you show, where you assume @solved and @tutti are assigned before you calculate the percentage expression, is not reliable.

A better way to do this is to put your query in a subquery, and then calculate the expression in the outer query using the aliases:

SELECT id, description, tipo, color, solved, tutti,
  (ROUND(solved/tutti,0)*100) as percentage
FROM (
    SELECT 
        items_type_catalog.id,
        items_type_catalog.description,
        items_header.tipo,
        items_type_catalog.color,
        COUNT(CASE WHEN (items_header.stato = 4 OR items_header.stato= 5) THEN 1 END) AS solved,
        COUNT(items_header.id) AS tutti
    FROM items_type_catalog 
    LEFT JOIN items_header 
      ON items_type_catalog.id=items_header.tipo AND items_header.progetto=1
    GROUP BY items_type_catalog.id, tipo
) AS t;