Parents without children
Given tables parent(id INT), child(id INT,parent_id INT), how do we find parents with no children? It's the All X for which there is no Y pattern, which can be written as an exclusion join...

SELECT parent.id
FROM parent
LEFT JOIN child ON parent.id = child.parent_id
WHERE child.parent_id IS NULL;

or with a NOT EXISTS subquery, which is logically equivalent to the exclusion join, but usually performs much slower:

SELECT parent.id AS ParentID
FROM parent
WHERE NOT EXISTS (
SELECT parent.id
FROM parent
JOIN child ON parent.ID = child.parent_id
);

Parties who Read More →

About the Abstract Factory
In the Abstract Factory Pattern, an abstract factory defines what objects the non-abstract or concrete factory will need to be able to create. The concrete factory must create the correct objects for it's context, insuring that all objects created by the concrete factory have been chosen to be able to work correctly Read More →