Beginner~1h
NULL Handling in SQL & DatabasesInterview Readiness Check
Three-valued logic, NULL = NULL trap, COALESCE, NULLIF, aggregate functions silently ignoring NULLs.
nullis nullcoalescenullifthree-valued logiccount null
Learning path
Try a sample question
SELECT * FROM users WHERE age = NULL; This returns 0 rows even though 50 users have age set to NULL. Why?
A.NULL values are stored separately and not scanned by WHERE clauses
B.In SQL's three-valued logic, NULL = NULL evaluates to UNKNOWN (not TRUE) — equality comparisons with NULL always return UNKNOWN. Use WHERE age IS NULL instead.
C.The column must be cast to INT before NULL comparison: WHERE CAST(age AS INT) = NULL
D.A missing index on age causes the NULL rows to be skipped
Sign up free to take the full check and see the answer with an explanation.
What this module tests
- Conceptual understanding of null handling fundamentals
- Trade-offs and design decisions in real system scenarios
- null and is null — applied knowledge
- Interview-style reasoning and explanation quality
Free to start
Take the NULL Handling check →
10 questions · Dimension scoring · AI coach feedback · Shareable result
Test your knowledge — free →No credit card · Takes 10–15 min
Frequently asked questions
- What does the NULL Handling check test?
- The check covers three-valued logic, null = null trap, coalesce, nullif, aggregate functions silently ignoring nulls. Questions span beginner to advanced difficulty and test both conceptual understanding and practical trade-off reasoning.
- What keywords should I know for NULL Handling?
- Key concepts include: null, is null, coalesce, nullif, three-valued logic, count null. These frequently appear in sql & databases interviews at product and infrastructure companies.
- How many questions are in the check?
- Each check has 10 questions — a mix of MCQ and short-answer. You get scored by dimension (e.g., scalability, consistency) so you know exactly where to focus.
- How long does it take to study NULL Handling?
- Most engineers spend around 1 hours on this module. The check itself takes about 10–15 minutes. You can retake it up to 3 times per day.
- What prerequisite knowledge do I need before NULL Handling?
- NULL Handling is a foundational module — no prerequisites needed.
- Is null, is null, coalesce covered?
- Yes. The NULL Handling module covers null, is null, coalesce among other key concepts. The check includes questions that reflect real sql & databases interview patterns.