..., bridges could collapse, spacecraft could fall out of the sky,
and so on, Do DBMSs Process Subqueries Correctly?
An Issue Paper by FFE Software, Inc.
In 1989, C. J. Date published an article detailing the EXISTS flaw in
ANSI SQL and most RDBMS implementations. He listed possible consequences
of this flaw:
While there is some hyperbole in Date's list, incorrect results and
processing can occur because of this error. This is a fundamental flaw
in ANSI SQL and infects most existing RDBMSs. FirstSQL is the only RDBMS
that has demonstrated correct EXISTS processing.
This paper examines the EXISTS flaw as documented by C. J. Date and the
larger ramifications of the problem. The first section describes the
original error pointed out by Date using an improved version of his
example. Unfortunately, Date didn't get it quite right. The EXISTS flaw
is symptomatic of larger problems that permeate most RDBMSs.
The middle section details the additional flaws in 3VL processing in ANSI
SQL. Some flaws are so insidious that they affect all subquery
processing in products, effectively eliminating reliable work-arounds.
The final section describes the FirstSQL solution to the EXISTS flaw and
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