4. Database Transactions Quiz
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Questions and Answers

What is a transaction in the context of database operations?

A transaction is a set of operations that are treated as a single unit, where either all operations are completed, or none at all.

What happens if a system hangs after the first SQL query of a transaction?

If the system hangs, the changes made by the first query may not be committed, leading to inconsistencies.

Explain the purpose of the commands 'SET AUTOCOMMIT FALSE' and 'SET AUTOCOMMIT TRUE'.

'SET AUTOCOMMIT FALSE' disables automatic updates after each statement, allowing manual control, while 'SET AUTOCOMMIT TRUE' re-enables this automatic behavior.

What is the role of the COMMIT command in a database transaction?

<p>The COMMIT command finalizes all changes made during a transaction, making them permanent.</p> Signup and view all the answers

What does the ROLLBACK command do in a transaction?

<p>The ROLLBACK command undoes all changes made during the current transaction, restoring the database to its previous state.</p> Signup and view all the answers

How does the isolation level 'Read Committed' function in transactions?

<p>'Read Committed' ensures that a transaction only sees changes that have been committed by other transactions.</p> Signup and view all the answers

What is the significance of the START TRANSACTION command in MySQL?

<p>The START TRANSACTION command signifies the beginning of a transaction, allowing for controlled execution of SQL statements.</p> Signup and view all the answers

What is meant by the term 'data isolation' in databases?

<p>Data isolation refers to the capability of transactions to operate independently, ensuring that uncompleted transactions do not affect others.</p> Signup and view all the answers

Study Notes

Transactions

  • Transactions are indivisible units of operations
  • Either all operations succeed or none do
  • Example: Transferring money
    • Funds are withdrawn from one account
    • Funds are added to another account
  • What happens if the system fails after the first step?

Database Transaction Example

  • Account information held by the bank
  • Example table:
    • AccountNo | Balance
    • LT9001 | 2000.0
    • LT9003 | 45.0

Transaction DB Example (2)

  • Example queries for transferring money
    • UPDATE Accounts SET Balance = Balance - 400 WHERE AccountNo = 'LT9001';
    • UPDATE Accounts SET Balance = Balance + 400 WHERE AccountNo = 'LT9003';
  • What happens if the program fails after the first query?

DB Solution

  • SET AUTOCOMMIT FALSE;
  • UPDATE Accounts SET Balance = Balance - 400 WHERE AccountNo = 'LT9001';
  • UPDATE Accounts SET Balance = Balance + 400 WHERE AccountNo = 'LT9003';
  • COMMIT;
  • SET AUTOCOMMIT TRUE;

DB Solution (2)

  • SET AUTOCOMMIT FALSE; - Turns off automatic commit after each statement
  • SET AUTOCOMMIT TRUE; - Turns on automatic commit after each statement
  • COMMIT; - Confirm changes
  • ROLLBACK; - Cancel all changes in a transaction
  • START TRANSACTION (BEGIN);- Indicates start of a transaction, commonly used in MySQL

Isolation

  • Different Database Management Systems (DBMS) support different isolation levels
  • Ensures data isolation in multi-user databases
  • Ensures unfinished transactions are not visible outside
  • Guarantees a transaction operates with specific data unaffected by other transactions
  • Ensures consistent data for the lifetime of a transaction.
  • Example isolation levels available:
    • Read committed
    • Repeatable read
    • Serializable
    • Read uncommitted

Isolation Level H2

  • Standard isolation level is Read Committed
  • Transaction data visible to others only after COMMIT
  • Consistency in repeated data reading within a transaction
  • Effect on views of results of other transactions

Task

  • Test INSERT, UPDATE, and DELETE statements in transactions (using ROLLBACK)
  • Test ALTER TABLE statements in transactions (adding a new column)
  • Test DROP TABLE statements in a transaction

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Transakcijos PDF

Description

Test your knowledge on database transactions, their properties, and examples of how transactions are implemented in SQL. This quiz covers key concepts like atomicity, failure scenarios, and the use of autocommit in databases.

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