Null (SQL): Difference between revisions

Revision as of 22:32, 2 November 2024

Revision as of 14:54, 7 November 2024

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would allow insertion of rows where author_last or author_first are {{NULL}} irrespective of how the table Authors is defined or what it contains. More precisely, a null in any of these fields would allow any value in the other one, even on that is not found in Authors table. For example, if Authors contained only {{code|('Doe', 'John')}}, then {{code|('Smith', NULL)}} would satisfy the foreign key constraint. [[SQL-92]] added two extra options for narrowing down the matches in such cases. If <code>MATCH PARTIAL</code> is added after the <code>REFERENCES</code> declaration then any non-null must match the foreign key, e.g. {{code|('Doe', NULL)}} would still match, but {{code|('Smith', NULL)}} would not. Finally, if <code>MATCH FULL</code> is added then {{code|('SmithDoe', NULL)}} would not match the constraint either, but {{code|(NULL, NULL)}} would still match it.

==Outer joins==