glossary
Null
Null is a SQL concept that helps learners reason about query meaning, result shape, and validation evidence before relying on a passing visible result.
Plain-language definition
Null means a practical SQL idea that changes how a learner should read tables, write queries, or interpret result shape.
In SQL Trail, the term is used only when it appears in lessons, feedback, reference pages, dataset notes, or mastery evidence.
Why it matters
Ignoring null can make a query look correct on a tiny visible dataset while failing hidden variants or edge cases.
The app therefore asks learners to connect definitions to output contracts, not just memorize vocabulary.
Tiny example
The small example below is a reading aid, not a full exercise. It shows how the concept changes the interpretation of a result.
SELECT customer_id FROM customers WHERE last_order_at IS NULL ORDER BY customer_id;| Concept | Practical check |
|---|---|
| Null | Name the row meaning, edge case, or validation risk before trusting the output. |