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Tip of the Day
Language: SQL Server
Expertise: Beginner
Jul 15, 1999

About tables

Question:
Please describe the basic components of a table in the relational model.

Answer:
A table has an intension and an extension. You can think of the intension of a table as the names of its columns, so the intension describes the thing the table is about. The extension of a table is the table's rows, or instances, examples, or samples of the table's intention— things that exist in the universe described by the domains of the table.

Domains are the conceptual pool of values from which attributes can be drawn, and are the glue that make comparisons possible in a database. Domains are what make operations make sense in attributes. We don't ask for the temperature of Orange, or the product of Nuts times Bolts because the comparisons make no sense because the domains are different.

The main thing for you to carry from this is that domains describe the universe of discourse, and the organization of domains into tables allows us to collect instances of the entity described by the domains. Operations defined on the domains are what let us do compares, make products, divisions, unions, differences and additions of relational facts.

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