SQL For Dummies by Allen G. Taylor

Author:Allen G. Taylor
Language: eng
Format: epub, pdf
ISBN: 9781119527091
Publisher: Wiley
Published: 2018-11-20T00:00:00+00:00

RULE BY COMMITTEE

The SQL-89 version of the standard specified the UNIQUE rule as the default, before anyone proposed or debated the alternatives. During development of the SQL-92 version of the standard, proposals appeared for the alternatives. Some people strongly preferred the PARTIAL rules and argued that those should be the only rules. These people thought that the SQL-89 (UNIQUE) rules were so undesirable that they wanted those rules considered a bug and the PARTIAL rules specified as a correction. Other people preferred the UNIQUE rules and thought that the PARTIAL rules were obscure, error-prone, and inefficient. Still other people preferred the additional discipline of the FULL rules. The issue was finally settled by providing all three keywords so users could choose whichever approach they preferred. SQL:1999 added the SIMPLE rules. Of course, the proliferation of rules makes dealing with nulls anything but simple. If UNIQUE, PARTIAL, or FULL isn't specified, the SIMPLE rules are followed.

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