Why this lesson matters

Understand transaction concept and reason about it with a concrete relational example. Transactions make a group of statements succeed or fail as one unit. COMMIT publishes the work, ROLLBACK discards it, and concurrency controls protect overlapping transactions from corrupting shared state.

How to reason about it

  • For Transaction Concept, the outcome to verify is: Run the statement against sample data and inspect the result before moving to the next case.
  • In Transaction Concept, keep this failure controlled: Reading a value and updating it later without an appropriate transaction or lock can lose updates when two sessions act concurrently.
  • Transaction Concept practice target: Transfer a value between two rows inside one transaction, force an error to test rollback, then simulate a concurrent update with row locking.
Diagram

Transaction lifecycle

Practical walkthrough

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

example.sqlsql
BEGIN;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
UPDATE accounts SET balance = balance + 100 WHERE id = 2;
COMMIT;

Practice it yourself

Practice

Transaction Concept exercise

Transfer a value between two rows inside one transaction, force an error to test rollback, then simulate a concurrent update with row locking.

  • 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