The set theory operations

The set theory operations are union, intersection, and minus (difference). Intersection is not a primitive relational algebra operator, because it can be written using the union and difference operators:

A∩B = ((A∪B)-(A-B))-(B-A)

The intersection and union are commutative:

A∩B=B∩A

A∪B=B∪A

For example, the query, give me all the customer IDs where the customer does not have a service assigned to him, could be written as follows:

π_{customer_id} customer-π_{customer_id} customer_service