NULL represents a missing or unknown value. Equality comparisons involving NULL do not evaluate to TRUE, so NULL values require special handling when JOINing or filtering data.
NULL values are common in real business databases and can cause unexpected JOIN results if their behavior is not understood.
A customer with a NULL region cannot match another table using a normal equality condition on Region.
SELECT
c.CustomerName,
r.RegionName
FROM Customers c
LEFT JOIN Regions r
ON c.RegionID = r.RegionID;Important in customer data, financial systems, HR databases, transaction systems, and data-quality analysis.
- NULL represents missing or unknown information.
- NULL does not equal NULL using =.
- LEFT JOIN can preserve rows with missing relationships.
- IS NULL is used to test for NULL.
- NULL behavior must be considered in JOIN logic.
Does NULL equal NULL in SQL?
No. NULL represents an unknown or missing value, so normal equality comparison does not return TRUE.
How can NULL be checked?
Use IS NULL or IS NOT NULL rather than the equality operator.