Why transactions ensure data consistency in DBMS Theory - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the sentence to explain what a transaction is in a database.

DBMS Theory

A transaction is a sequence of operations that [1] the database from one consistent state to another.

Drag options to blanks, or click blank then click option'

Aleaves

Bchanges

Cdeletes

Dignores

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing 'ignores' or 'deletes' which do not describe transactions correctly.

A transaction changes the database from one consistent state to another, ensuring data integrity.

2fill in blank

medium

Complete the sentence to describe the property that ensures all parts of a transaction succeed or fail together.

DBMS Theory

The property that ensures a transaction is all-or-nothing is called [1].

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AIsolation

BDurability

CAtomicity

DConsistency

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing Atomicity with Consistency or Isolation.

Atomicity means the transaction is indivisible; it either fully happens or does not happen at all.

3fill in blank

hard

Fix the error in the sentence about transaction isolation.

DBMS Theory

Transaction isolation ensures that concurrent transactions [1] each other's intermediate states.

Drag options to blanks, or click blank then click option'

Ahide

Bignore

Cshare

Dsee

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing 'see' or 'share' which allow exposure of partial data.

Isolation means transactions hide their intermediate states from others to prevent inconsistent data.

4fill in blank

hard

Fill both blanks to complete the explanation of the consistency property.

DBMS Theory

Consistency ensures that a transaction [1] the database from one valid state to another and [2] all defined rules.

Drag options to blanks, or click blank then click option'

Atransforms

Bignores

Cfollows

Dbreaks

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing 'ignores' or 'breaks' which contradict consistency.

Consistency means the database moves between valid states and follows all rules like constraints.

5fill in blank

hard

Fill all three blanks to complete the sentence about durability in transactions.

DBMS Theory

Durability means once a transaction is [1], its changes are [2] in the database and will [3] even if the system crashes.

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Acommitted

Bpermanently saved

Cpersist

Drolled back

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing 'rolled back' which means undoing changes.

Durability guarantees that committed changes are permanently saved and persist despite failures.

Practice

1. What is the main reason transactions ensure data consistency in a database?

easy

A. They speed up the database queries.

B. They allow multiple users to edit the same data at once without control.

C. They make sure all parts of a task complete successfully or none do.

D. They store data in multiple formats.

Solution

1. Step 1: Understand the role of transactions

   Transactions group multiple operations into one unit that either fully succeeds or fully fails.

2. Step 2: Connect to data consistency

   This all-or-nothing approach prevents partial updates that could corrupt data.

3. Final Answer:

   They make sure all parts of a task complete successfully or none do. -> Option C

4. Quick Check:

   All-or-nothing = Data consistency [OK]

Hint: Remember: transactions are all-or-nothing actions [OK]

Common Mistakes:

• Thinking transactions speed up queries
• Believing transactions allow uncontrolled edits
• Confusing data formats with consistency

2. Which of the following is the correct SQL command to start a transaction?

easy

A. RUN TRANSACTION;

B. START QUERY;

C. OPEN DATABASE;

D. BEGIN TRANSACTION;

Solution

1. Step 1: Identify the standard SQL command

   The standard command to begin a transaction is BEGIN TRANSACTION;.

2. Step 2: Eliminate incorrect options

   Other options like START QUERY; or OPEN DATABASE; are not valid SQL commands for transactions.

3. Final Answer:

   BEGIN TRANSACTION; -> Option D

4. Quick Check:

   Start transaction = BEGIN TRANSACTION [OK]

Hint: Transaction always starts with BEGIN TRANSACTION [OK]

Common Mistakes:

• Using START QUERY instead of BEGIN TRANSACTION
• Confusing transaction start with database open
• Using RUN TRANSACTION which is invalid

3. Consider this sequence of SQL commands:
BEGIN TRANSACTION;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
UPDATE accounts SET balance = balance + 100 WHERE id = 2;
COMMIT;
What is the main result of this transaction?

medium

A. Both updates are saved together or none if error occurs.

B. Only the first update is saved, second is ignored.

C. The transaction rolls back automatically after updates.

D. The updates happen but data may become inconsistent.

Solution

1. Step 1: Analyze the transaction commands

   The commands start a transaction, perform two updates, then commit the changes.

2. Step 2: Understand commit effect on data consistency

   Commit saves all changes together. If any error occurs before commit, none are saved, ensuring consistency.

3. Final Answer:

   Both updates are saved together or none if error occurs. -> Option A

4. Quick Check:

   Commit = all-or-nothing save [OK]

Hint: COMMIT saves all changes together [OK]

Common Mistakes:

• Thinking only first update saves
• Assuming automatic rollback without error
• Believing updates cause inconsistency

4. A developer writes this code:
BEGIN TRANSACTION;
UPDATE inventory SET quantity = quantity - 5 WHERE product_id = 10;
-- Missing COMMIT here
UPDATE sales SET total = total + 100 WHERE sale_id = 20;
What is the main problem with this transaction?

medium

A. The transaction will never start.

B. The changes may not be saved due to missing COMMIT.

C. The updates will cause syntax errors.

D. The transaction will auto-commit after each update.

Solution

1. Step 1: Identify missing transaction command

   The code starts a transaction but does not include a COMMIT command to save changes.

2. Step 2: Understand effect of missing COMMIT

   Without COMMIT, changes remain uncommitted and may be lost if connection closes or rollback occurs.

3. Final Answer:

   The changes may not be saved due to missing COMMIT. -> Option B

4. Quick Check:

   Missing COMMIT = no save [OK]

Hint: Always end transactions with COMMIT or ROLLBACK [OK]

Common Mistakes:

• Assuming transaction never starts
• Expecting syntax errors from missing COMMIT
• Believing auto-commit happens without command

5. A bank uses transactions to transfer money between accounts. If the system crashes after deducting money from one account but before adding it to another, how do transactions help maintain data consistency?

hard

A. They roll back all changes so no partial update remains.

B. They automatically retry the failed operation endlessly.

C. They save the deducted amount and ignore the addition.

D. They allow partial updates to keep data moving.

Solution

1. Step 1: Understand partial failure risk in money transfer

   If money is deducted but not added, data becomes inconsistent and incorrect.

2. Step 2: Explain transaction rollback mechanism

   Transactions ensure either all steps succeed or all fail. On crash, rollback undoes partial changes.

3. Step 3: Connect rollback to data consistency

   Rollback prevents partial updates, keeping data accurate and consistent after failure.

4. Final Answer:

   They roll back all changes so no partial update remains. -> Option A

5. Quick Check:

   Rollback = no partial updates [OK]

Hint: Rollback undoes partial changes after failure [OK]

Common Mistakes:

• Thinking partial updates are allowed
• Believing system retries endlessly automatically
• Assuming partial data is saved anyway