null value

1. A value that means not known or not applicable. A null value is distinct from a string of blanks or from a value of zero. Database columns and program variables can have null values. In 4GL, this marker referred to by the keyword NULL. To test for a null value, use the IS NULL and IS NOT NULL operators.

See also Boolean operator.

2. In other contexts, null is casually used to mean empty; for example, a character string with zero length is sometimes called the null string. This can lead to confusion because an empty string (the string ) has a specific non-null value, distinct from a NULL string. An empty string has a definite length (zero) while a NULL character value has an unknown length and value.

See also blank space, string.