left-join.md

SELECT category.CategoryName
FROM category
LEFT JOIN products
    ON category.category_id = products.category_id
WHERE products.category_id IS NULL;

############################################# SCHEMA #############################################
define
product sub entity,
    key product_name,
    plays product-assignment;
category sub entity,
    key category_name,
    plays assigned-category;

category-assignment sub relation,
    relates assigned-category,
    relates product-assignment;

category_name sub attribute, datatype string;
product_name sub attribute, datatype string;

############################################# DATA #############################################
insert
$pl isa product, has product_name "Lobster";
$pa isa product, has product_name "Almond";
$pc isa product, has product_name "Cashew";

$cb isa category, has category_name "Bean";
$cs isa category, has category_name "Seafood";
$cn isa category, has category_name "Nut";

(assigned-category: $cs, product-assignment: $pl) isa category-assignment;
(assigned-category: $cn, product-assignment: $pa) isa category-assignment;
(assigned-category: $cn, product-assignment: $pc) isa category-assignment;

############################################# QUERY #############################################
match
$c isa category,
    has category_name $cn;
not {
    ($c, $x) isa category-assignment;
};
get $cn;