Why this lesson matters

Understand delete and reason about it with a concrete relational example. INSERT creates rows, UPDATE changes selected rows and DELETE removes them. The WHERE clause and constraints determine whether a write affects exactly the intended data.

How to reason about it

  • For DELETE, the outcome to verify is: Run the statement against sample data and inspect the result before moving to the next case.
  • In DELETE, keep this failure controlled: An UPDATE or DELETE without a verified predicate can modify an entire table; a write without transaction/error handling can leave multi-step work inconsistent.
  • DELETE practice target: Insert a sample row, update one field by primary key, then delete only a deliberately chosen row and verify row counts after each step.

Practical walkthrough

In the DELETE walkthrough: Run the statement against sample data and inspect the result before moving to the next case.

example.sqlsql
DELETE FROM sessions
WHERE expires_at < CURRENT_TIMESTAMP;

Practice it yourself

Practice

DELETE exercise

Insert a sample row, update one field by primary key, then delete only a deliberately chosen row and verify row counts after each step.

  • Record the expected result before execution
  • Test one valid path and one lesson-specific failure path
  • Explain in two lines which boundary owns the decision

Summary