How to Use CANDIDATE Keys (127846)
The information in this article applies to:
- Microsoft Visual FoxPro for Windows 3.0
This article was previously published under Q127846 SUMMARY
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
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
"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.
| Modification Type: | Major | Last Reviewed: | 2/12/2000 |
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| Keywords: | KB127846 |
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