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Questions and Answers
What is a transaction?
A collection of operations that form a single logical unit of work.
A transaction to transfer ₱1000 from Aldous to Brendon involves multiple steps including reading and writing to their accounts. The first step is to read Aldous's account, the second step is to _____.
subtract ₱1000 from Aldous's account
Isolation ensures that the data used during the execution of a transaction can be used by another transaction simultaneously.
False
Which property of transactions ensures that once changes are committed, they cannot be undone or lost?
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What happens if a transaction fails after some changes have been made?
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What is meant by the term 'transaction log'?
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Match the following transaction properties with their definitions:
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Study Notes
Transaction
- A collection of operations that form a single logical unit of work.
- Example: A customer purchasing two pairs of shoes from a cashier.
- Involves multiple database operations like writing a new invoice, reducing inventory, and updating sales.
- If one operation fails, all are undone.
- A database request is equivalent to a single SQL statement in an application program or transaction.
- Involves multiple database requests for each SQL statement.
Properties of transactions
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Atomicity: Requires all operations of a transaction to be completed. If one fails, the entire transaction is aborted.
- Example: Transaction with 4 SQL requests must all complete.
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Consistency: Ensures that only valid data is written to the database.
- Data must follow all rules and constraints.
- If invalid data is written, the database reverts to its previous state.
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Isolation: Data accessed during a transaction cannot be used by another transaction until the first one is completed.
- Example: Two people using the same ATM card simultaneously. The first connection locks the account until their session ends.
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Durability: Ensures that once changes are committed, they cannot be undone or lost.
- Changes persist even in case of failures.
Real-Life Transaction: Fund Transfer
- Transferring P1000 from Aldous to Brendon's bank account involves the following steps:
- Read Aldous's account.
- Deduct P1000 from Aldous's account.
- Write changes to Aldous's account.
- Read Brendon's account.
- Add P1000 to Brendon's account.
- Write changes to Brendon's account.
Transaction Properties in Fund Transfer
- Atomicity: If the transaction fails after updating Aldous's account but before updating Brendon's, the updates are not reflected in the database.
- Consistency: The sum of Aldous's and Brendon's accounts remains unchanged. Any inconsistencies due to access during the transaction are prevented.
- Isolation: Transactions run sequentially, preventing other transactions from accessing the accounts during transfer.
- Durability: The transfer is permanent even after system failures.
Database Consistent State
- Satisfies constraints specified in the database schema.
- A table with a 'gender' column that only accepts 'Male' or 'Female' values.
- The database will reject any attempts to enter invalid values like 'Person'.
Transaction Log
- Used by DBMS to track transactions that update the database.
- Helps restore the database to a consistent state in case of system failure.
SQL Transactional Commands
- BEGIN TRANSACTION: Starts a new transaction.
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Description
Test your knowledge on database transactions and their properties such as atomicity, consistency, and isolation. This quiz will cover essential concepts related to logical units of work and the importance of transaction management in databases.
