null

Null in computing and information systems denotes the absence of a value or the lack of a meaningful object. It is not the same as zero, an empty string, or the boolean value false. Null can indicate that data is missing, not yet known, or not applicable, and it often requires explicit handling to avoid errors in software and queries.

Null in programming languages usually marks the absence of an object or value. Java and C# use null as a reference that points to no object; C and C++ provide NULL (macro) or, in modern C++, nullptr. Python uses None; JavaScript has null and undefined, with undefined representing a not-yet-assigned value. SQL databases designate missing information as NULL, and arithmetic with NULL typically yields NULL.

SQL's NULL participates in three-valued logic; comparisons with NULL do not yield true or false. To test for missing data you use IS NULL or IS NOT NULL. Aggregate functions and predicates can behave differently when NULL values are present, requiring careful handling in queries and data processing.

In mathematics, 'null' commonly refers to the empty set, denoted ∅, or to a null space or null vector in linear algebra, where a vector of all zeros lies in the null space of a linear transformation. In logic and philosophy, 'null' is used more loosely to indicate absence, but precise definitions depend on the formal system in use.

Handling nulls is a major consideration in software design. Frequent null-related errors include null reference exceptions. Some languages offer alternatives such as option types or nullable annotations to reduce null handling risks, promoting safer APIs and clearer contracts about when a value may be absent.