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Data & Analytics · Foundational

NULL

A marker meaning a value is missing or unknown; it is not zero, an empty string or false.

Definition

NULL represents the absence of a value. Comparisons with NULL return UNKNOWN, so you must test with IS NULL or IS NOT NULL. Most aggregate functions ignore NULLs, though COUNT(*) counts every row.

Example

SELECT * FROM Customers WHERE Phone IS NULL;

Exam tip

'WHERE col = NULL' never returns rows; use IS NULL. COUNT(col) skips NULLs while COUNT(*) does not.

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