July 2010
Intermediate to advanced
840 pages
16h 33m
English
Content preview from Joe Celko's SQL for Smarties, 3rd Edition
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2.11. Key Types
The logical and physical keys for a table can be classified by their behavior and their source. Table 2.1 is a quick table of my classification system.
Table 2.1. Classification System for Key Types
Natural Artificial "Exposed Surrogate
Physical
Locator"
=====================================================================
Constructed from attributes |
in the reality |
of the data model | Y N N Y
|
Verifiable in reality | Y N N N
|
Verifiable in itself | Y Y N N
|
Visible to the user | Y Y Y N
|
2.11.1. Natural Keys
A natural key is a subset of attributes that occur in a table and act as a unique identifier. The user sees them. You can go to external reality and verify them. Examples of natural keys include the UPC codes on ...
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