null

This sort of effect can be kludged via magic numbers like -1 or other arbitary values, but it's better if your language of choice does it internally, eliminating any possible ambiguity.

Some languages such as Perl give it different names, like undef. Semantically this suits better in a situation where you want to express the lack of a value, while null in a C context generally indicates a meaningless value.

See also: pointers, code, and C.

In a (relational) database, NULL indicates that a field does not have a value.

Usual arithmetic rules do not apply to NULL values:

• Almost all calculations involving NULL values gives NULL as result, for example 5 + NULL has the result NULL. String concatenation does not always work this way, for example the Oracle operator for concatenating strings, ||, treats NULL as an empty string.
• No value is equal to NULL, not even NULL itself! Therefore there is a special <value> IS NULL operator that is used to test for NULL values.

Note: On some database servers, for example Oracle 8, an empty string is equal to null, but on others, for example Microsoft SQL Server, an empty string is not equal to null (but that is configurable, IIRC).

Null (?), a. [L. nullus not any, none; ne not + ullus any, a dim. of unus one; cf. F. nul. See No, and One, and cf. None.]

Of no legal or binding force or validity; of no efficacy; invalid; void; nugatory; useless.

Faultily faultless, icily regular, splendidly null, Dead perfection; no more. Tennyson.

© Webster 1913.

Null, n.

1.

Something that has no force or meaning.

2.

That which has no value; a cipher; zero.

Bacon.

Null method Physics., a zero method. See under Zero.

© Webster 1913.

Null, v. t. [From null, a., or perh. abbrev. from annul.]

To annul.

Milton.

© Webster 1913.

Null, n. [Etymol. uncertain.]

One of the beads in nulled work.

© Webster 1913.