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Q136851: Cases Where Failed ALTER TABLE Doesn’t Revert

Article: Q136851
Product(s): Microsoft FoxPro
Version(s): WINDOWS:3.0
Operating System(s): 
Keyword(s): 
Last Modified: 15-FEB-2000

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The information in this article applies to:

- Microsoft Visual FoxPro for Windows, version 3.0 
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SUMMARY
=======

This article describes circumstances that change the table structure even if the
ALTER TABLE command fails.

MORE INFORMATION
================

In general, any failure reported by either the ALTER TABLE command or the MODIFY
STRUCTURE dialog box should result in the source table remaining unchanged from
its state prior to the action.

One exception to this is when a new primary key is created or an existing key is
promoted to being a primary key. An error message is displayed if the key
creation fails because Visual FoxPro detects a duplicate key. However, all other
changes to the table remain intact.

The SET SAFETY ON/OFF command also affects some of the validation performed by
the ALTER TABLE and MODIFY STRUCTURE operations. The following information
applies to the table validation rules, table validation text, field validation
rules, field validation text, and field default values.

Regardless of the setting of safety, the syntax of the information entered is
validated. However, if SAFETY is ON, then the information is also evaluated
before any attempt is made to alter the table. Therefore, setting SAFETY ON
provides an extra level of validation that can occur even if the the NOVALIDATE
clause is used to avoid a lenthgy rule verification phase.

Example: For a field rule for FIELD1 where MEMVAR1 is not in scope:

     ALTER TABLE Table1 ALTER Field1 CHECK Field1 !=memvar1 NOVALIDATE

This succeeds if SAFETY is OFF, but fails evaluation because MEMVAR1 is not in
scope.

NOTE: If a user-defined function (UDF) is recognized during the validation phase,
the function is not called and the validation passes.

Additional query words: VFoxWin sql

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

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