Firebird Null Guide

Summary

NULL in a nutshell:

•NULL means unknown.

•If NULL figures in an expression, most of the time the entire expression becomes NULL.

•In aggregate functions only non-NULL fields are involved in the computation. Exception:

COUNT(*).

•UDFs sometimes convert NULL <-> non-NULL in a seemingly random manner.

•If the test expression of an if statement is NULL, the then block is skipped and the else block executed.

• To find out if A is NULL, use “A is (not) null”.

•The COALESCE and *NVL functions can convert NULL to a value.

•Assigning NULL is done like assigning values: with “A = NULL” or an insert list.

Remember, this is how NULL works in Firebird SQL. There may be (subtle) differences with other RDBMSes.

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Firebird Null Guide

Appendix A:

Document history

The exact file history is recorded in the manual module in our CVS tree; see http://sourceforge.net/ cvs/?group_id=9028

Revision History
0.1	8 April 2005	PV	First edition.
0.2	15 April 2005	PV	Mentioned that Fb 2.0 legalises “A = NULL” comparisons.
			Changed text in “Testing if something is NULL”.
			Slightly altered “Dealing with NULLs” section.

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Firebird Null Guide

Appendix B:

License notice

The contents of this Documentation are subject to the Public Documentation License Version 1.0 (the “License”); you may only use this Documentation if you comply with the terms of this License. Copies of the License are available at http://www.firebirdsql.org/pdfmanual/pdl.pdf (PDF) and http:// www.firebirdsql.org/manual/pdl.html (HTML).

The Original Documentation is titled Firebird Null Guide.

The Initial Writer of the Original Documentation is: Paul Vinkenoog.

Copyright (C) 2005. All Rights Reserved. Initial Writer contact: paulvink at users dot sourceforge dot net.

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