SQL & Databases Masterclass Course
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SQL Lesson 8: A short note on NULLs
Introduction & Core Concept
In relational databases, NULL represents a state of missing, unknown, unassigned, or undefined data.
It is crucial to understand that NULL is not equivalent to zero (0), nor is it equivalent to an empty text string (''). Zero is a specific number, and an empty string is a valid text value. NULL means the value is completely absent.
Because NULL represents an unknown state, standard mathematical comparison operators (like = NULL or != NULL) always evaluate to unknown and fail to return rows. Instead, you must use specialized null-checking operators:
IS NULL: Evaluates to true if the attribute is empty/missing.IS NOT NULL: Evaluates to true if the attribute contains populated data.Why & Where We Use It
LEFT JOIN operations.Real-World Example
Suppose you run a LEFT JOIN connecting registered SpotifyLite artists to their uploaded songs. To isolate specifically the artists who have *never* uploaded a song, you filter the results using WHERE s.id IS NULL.
Best Practices: What to Do & What NOT to Do
COALESCE(column, 'Default') to replace potential NULL values with clean, readable fallback strings before presenting reports to executives.WHERE column = NULL. It is a classic beginner trap that will silently return zero rows without throwing a syntax error!Syntax & Pro Tips
SELECT name, genre
FROM artists a
LEFT JOIN songs s ON a.id = s.artist_id
WHERE s.id IS NULL;