DOCUMENT:Q127846  12-FEB-2000  [foxpro]
TITLE   :How to Use CANDIDATE Keys
PRODUCT :Microsoft FoxPro
PROD/VER:WINDOWS:3.0
OPER/SYS:
KEYWORDS:

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The information in this article applies to:

 - Microsoft Visual FoxPro for Windows, version 3.0 
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SUMMARY
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A CANDIDATE key is an index expression that does not allow duplicate or NULL
values. This articles provides examples that demonstrate the use of candidate
keys.

MORE INFORMATION
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By definition, a candidate key is a single field or a composite expression that
meets the requirements of a primary key. It is a database design requirement
that tables have one and only one primary key. A table might however contain
several unique identifiers. Once the primary key has been identified, other
unique keys are referred to as candidate keys.

In Visual FoxPro, primary keys are used to reinforce data integrity, and are
therefore only available for tables included in a database. Free tables on the
other hand can have candidate keys. Candidate keys can be used to ensure that a
record is unique.

Following are two scenarios that might require the use of a candidate key in
addition to a primary key.

Employee table: you have a table that records information about employees. Each
employee can be identified by an employee identification number (primary key)
and also by a social security number (candidate key).

Medical system: a table records patient information. Two fields can uniquely
identify each patient: a medical record number (unique key) and a social
security number (candidate key). The primary key provides a more intuitive way
to search the data.

REFERENCES
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"Developer's Guide", Visual FoxPro version 3.0, chapter 7, Working with Tables.

"A Guide to the SQL Standard", C.J. Date, p. 44, Addison-Wesley.

Additional query words: VFoxWin 3.00

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Keywords          :  
Technology        : kbVFPsearch kbAudDeveloper kbVFP300
Version           : WINDOWS:3.0

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