What is a database index?

Answer

SQL is a declarative language for querying and changing relational data. Correctness starts with joins, constraints, and transactions; performance comes from inspecting real query plans and indexing the access pattern.

Example

Consider a production SQL change: define the expected behavior and failure modes first, implement the smallest observable change, then verify it with a focused test or measurement. The right implementation depends on the system boundary and its constraints.

Practical considerations

1. Choose the approach from the requirement and constraints, not from habit.
2. Include validation, error handling, and cleanup where the boundary requires them.
3. Verify the observable result with focused tests or measurement.

Follow-up prompts

• What failure mode would you expect if this were implemented incorrectly?
• How would you test this behaviour?
• What changes when the feature must scale to a larger application or team?

In practice

For this easy-level topic, make assumptions explicit, choose the smallest safe implementation, and verify the behavior at the relevant boundary.