Interpret NULL as missing or unknown information, use IS NULL correctly, and predict how three-valued comparisons affect filtered results.
Beginner14 min read
SQL Fundamentals lessonRelational data foundationsPractice
UNTIMED COACHING
Practice before the pressure
Use feedback to correct the model, not merely memorize an option position.
GUIDED PRACTICE
Practice the lesson questions
Answer normal lesson questions without a timer. Every answer includes an explanation, and incorrect answers can be tried again before continuing.
20 questions balanced across the lesson's key topics
At least four explanatory diagrams or tables
Retry freely, without a timer or score
CONCEPT MODELS
See the lesson as a system
Use these visual guides to connect the key ideas before answering the questions.
Connect the roles in Work with NULL and Unknown ValuesKeep null value, comparison outcome, unknown result, and null predicate as separate roles.Verify Optional columnConnect optional column with its topic-specific inspection, expected outcome, and protected database boundary.