Build a Practical Transactions and Concurrency Example in SQL and Relational Databases

Define the build target

For Transactions and Concurrency, transfer a value between two rows in one transaction and prove a failed second step rolls back the first. Build the boundary case using this implementation lens: Use relational tables, keys, transactions, SQL result sets, constraints, query plans, and database state as the concrete model.

Keep the Transactions and Concurrency build centered on these technical constraints: BEGIN/COMMIT/ROLLBACK. Atomicity. Apply them through this path lens: Use relational tables, keys, transactions, SQL result sets, constraints, query plans, and database state as the concrete model. Use relational tables, keys, transactions, SQL result sets, constraints, query plans, and database state as the concrete model.

Implement the core behavior

Implement Transactions and Concurrency around the module artifact—a working transactions and concurrency example with an explicit success and failure check—and keep the implementation specific to this path context: Use relational tables, keys, transactions, SQL result sets, constraints, query plans, and database state as the concrete model.

BEGIN;
UPDATE accounts SET balance = balance - 50 WHERE id = 1;
UPDATE accounts SET balance = balance + 50 WHERE id = 2;
-- Verify both rows before committing.
SELECT id, balance FROM accounts WHERE id IN (1, 2) ORDER BY id;
COMMIT;

psql -f transfer.sql

Both account changes commit together; a failure before COMMIT can be rolled back.

practice/\n├── README.md\n├── transactions-and-concurrency-build.sql\n└── evidence/\n    └── expected-result.txt

Run the complete path

Run one realistic Transactions and Concurrency case end to end and record the required evidence: the relevant output, test, log, query result, or rendered state for Transactions and Concurrency. Interpret the result through this path context: Use relational tables, keys, transactions, SQL result sets, constraints, query plans, and database state as the concrete model.

Change one meaningful condition

Modify one condition central to Transactions and Concurrency using this path context: Use relational tables, keys, transactions, SQL result sets, constraints, query plans, and database state as the concrete model. Predict the new result before rerunning the same workflow.

Verify the artifact

Your deliverable is a working transactions and concurrency example with an explicit success and failure check.

Common mistakes to avoid

• Autocommit causes partial update.
• Transaction is too broad.
• Deadlock not retried safely.
• Read-modify-write race.