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Short Answer Closure in relational algebra has nothing to do with the concept of a key. A relational variable holds a relation so long as that relation has the following properties: Tuples are not ordered - order holds no implicit meaning Tuples are distinct - there are no duplicates Attributes are not ordered - order holds no implicit meaning Attributes ...


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When Codd defined the relational model he defined a set of operators which could be applied to relations. In specifying a relational algebra, much like specification of an integer algebra, we are able to use symbols in place of relations to solve queries. These operators are subject to the same algebraic properties that integer algebra operators (+, -, *, ...


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I strongly feel you can be successful as a DBA without taking a formal course in relational algebra, just like you can be a successful programmer without taking a formal course in discrete math. The need to take a course in relational algebra would very much depend on your career path/goals. See this question on Programming StackExchange. The OP wanted to ...


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Your current diagram is great as long as everyone only has one address, and a max of two phone numbers (apart from a fax). One simple way this schema falls apart is if you have a client with a street address for their location, but a PO Box for their mailing address. This is a fairly common setup for rural communities in the United States. When I lived ...


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What you have in the image is great. It's a place to put contact information for both of those other entities. I don't even understand why you'd need to have a contact type... You don't want to create two tables that contain the same columns under almost any circumstances.


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I want to persist contact details for persons. Since persons could include both staff and clients, the natural step would be to have a single table of contacts for all persons. As oNare suggested in a comment above, just create a contactType field in which you can assign either person type. This solution has the benefit of adding in future, if a need ...



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