SELECT count(*) FROM Ref_colors WHERE color_code NOT IN ( SELECT color_code FROM products )
Count the number of colors that are not used in any products.
SELECT count ( * ) FROM Ref_colors WHERE color_code NOT IN ( SELECT color_code FROM products )
select count ( * ) from ref_colors where color_code not in ( select color_code from products )
Count the number of colors that are not used in any products .