NULL represents a missing or unknown value. Because NULL is not equal to another NULL using the normal equals operator, JOIN conditions involving NULL require careful handling.
NULL values are common in real-world databases and can cause unexpected results when relationships are incomplete.
Identify customers whose optional account reference does not have a matching value.
SELECT
c.CustomerName,
a.AccountNumber
FROM Customers c
LEFT JOIN Accounts a
ON c.AccountID = a.AccountID;NULL-aware JOIN logic is important in data quality, reconciliation, customer records, and incomplete operational data.
- NULL represents missing or unknown information.
- NULL does not equal NULL using =.
- OUTER JOINs can produce NULL values.
- IS NULL and IS NOT NULL are used to test NULL values.
- NULL handling is important for accurate analysis.
Does NULL equal NULL in SQL?
No. NULL represents an unknown or missing value, so normal equality comparisons with NULL do not return TRUE.
How do you check for NULL values?
Use IS NULL or IS NOT NULL rather than the equals operator.