January 2019
Beginner
556 pages
14h 19m
English
Content preview from Learning PostgreSQL 11 - Third Edition
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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:
πcustomer_id customer-πcustomer_id customer_service
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