SQL TutorialLesson 11Free preview

SQL Null Values

This lesson is part of SQL · All levels

Summary

NULL means an unknown or unavailable value, not a zero or empty string. Any comparison with NULL, for example = NULL, always yields UNKNOWN so no row is selected. To check NULL use IS NULL and IS NOT NULL.

Key points

NULL is an unknown value; it differs from 0, an empty string, or FALSE.
Do not use = NULL or <> NULL; use IS NULL / IS NOT NULL.
COUNT(column) ignores NULL, whereas COUNT(*) counts all rows.
Arithmetic operations with NULL yield NULL, for example NULL + 5 = NULL.
Column with constraint NOT NULL should not store NULL.

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