ACID properties mental model in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the 'A' in ACID stand for in database transactions?

The 'A' stands for Atomicity. It means a transaction is all or nothing: either all steps complete successfully, or none do.

beginner

Explain the 'C' in ACID properties.

The 'C' stands for Consistency. It means the database moves from one valid state to another, following all rules and constraints.

intermediate

What does 'I' represent in ACID and why is it important?

'I' stands for Isolation. It ensures that transactions run independently without interfering with each other, like people working in separate rooms.

beginner

Describe the 'D' in ACID properties.

'D' means Durability. Once a transaction is committed, its changes are permanent, even if the system crashes immediately after.

intermediate

Why is the ACID model important for databases?

ACID ensures reliable transactions, preventing errors like partial updates or data corruption, so users trust the data is correct and safe.

Which ACID property ensures a transaction is completed fully or not at all?

A Durability

B Consistency

C Atomicity

D Isolation

Atomicity means the transaction is all or nothing.

Which property guarantees that once a transaction commits, changes are permanent?

A Isolation

B Durability

C Consistency

D Atomicity

Durability ensures changes survive system failures.

What does Isolation prevent in concurrent transactions?

A Interference between transactions

B Data loss

C Partial updates

D Invalid data states

Isolation keeps transactions from affecting each other.

Consistency in ACID means:

A Transactions run independently

B Transactions are all or nothing

C Changes are permanent

D Database rules are always followed

Consistency means the database stays valid after transactions.

If a transaction fails halfway, which ACID property ensures no partial changes remain?

A Atomicity

B Isolation

C Durability

D Consistency

Atomicity rolls back partial changes on failure.

Describe each ACID property using a simple real-life example.

Think about how a bank transaction works.

Why is it important for a database to follow ACID properties?

Consider what happens if data gets corrupted or lost.

Practice

1. Which ACID property ensures that a transaction is completed fully or not at all, like finishing a whole recipe or not starting it?

easy

A. Isolation

B. Consistency

C. Atomicity

D. Durability

Solution

1. Step 1: Understand Atomicity meaning

   Atomicity means a transaction is all-or-nothing, like a recipe that must be fully done or not started.

2. Step 2: Compare with other properties

   Consistency means data rules are followed, Isolation means transactions don't interfere, Durability means changes are saved permanently.

3. Final Answer:

   Atomicity -> Option C

4. Quick Check:

   All or nothing = Atomicity [OK]

Hint: All or nothing means Atomicity [OK]

Common Mistakes:

• Confusing Consistency with Atomicity
• Thinking Isolation means all-or-nothing
• Mixing Durability with Atomicity

2. Which of the following SQL transaction commands is used to make all changes permanent, supporting the Durability property?

easy

A. ROLLBACK

B. COMMIT

C. SAVEPOINT

D. BEGIN TRANSACTION

Solution

1. Step 1: Identify command for saving changes

   COMMIT saves all changes permanently, ensuring Durability.

2. Step 2: Understand other commands

   ROLLBACK undoes changes, SAVEPOINT marks a point to rollback to, BEGIN TRANSACTION starts a transaction.

3. Final Answer:

   COMMIT -> Option B

4. Quick Check:

   Permanent save = COMMIT [OK]

Hint: COMMIT makes changes permanent [OK]

Common Mistakes:

• Choosing ROLLBACK to save changes
• Confusing SAVEPOINT with COMMIT
• Thinking BEGIN TRANSACTION saves data

3. Consider two transactions running at the same time: Transaction 1 adds 100 to an account balance, Transaction 2 subtracts 50. Which ACID property ensures the final balance is correct as if transactions ran one after another?

medium

A. Isolation

B. Consistency

C. Atomicity

D. Durability

Solution

1. Step 1: Understand concurrent transactions

   When two transactions run together, Isolation ensures they don't interfere and produce correct results.

2. Step 2: Differentiate from other properties

   Atomicity is about all-or-nothing, Consistency is about data rules, Durability is about saving changes permanently.

3. Final Answer:

   Isolation -> Option A

4. Quick Check:

   Transactions separated = Isolation [OK]

Hint: No interference means Isolation [OK]

Common Mistakes:

• Mixing Consistency with Isolation
• Thinking Atomicity handles concurrency
• Confusing Durability with Isolation

4. A developer notices that after a system crash, some committed transactions are lost. Which ACID property is likely violated, and what should be fixed?

medium

A. Atomicity; ensure transactions are all-or-nothing

B. Consistency; fix data validation rules

C. Isolation; improve transaction locking

D. Durability; improve data saving and recovery

Solution

1. Step 1: Identify property related to crash recovery

   Durability ensures committed transactions survive crashes and are saved permanently.

2. Step 2: Match problem to property

   Lost committed transactions after crash means Durability is violated and saving/recovery must be improved.

3. Final Answer:

   Durability; improve data saving and recovery -> Option D

4. Quick Check:

   Crash survival = Durability [OK]

Hint: Lost data after crash means Durability issue [OK]

Common Mistakes:

• Confusing Atomicity with crash recovery
• Thinking Isolation fixes lost data
• Mixing Consistency with durability problems

5. A bank system must ensure that transferring money from one account to another never leaves the database in an incorrect state, even if the system crashes or many users act at once. Which combination of ACID properties best guarantees this?

hard

A. Atomicity, Consistency, and Durability

B. Consistency and Isolation

C. Isolation and Durability

D. Atomicity and Durability

Solution

1. Step 1: Identify properties needed for correctness and crash safety

   Atomicity ensures full transfer or none, Consistency keeps data valid, Durability saves changes permanently.

2. Step 2: Understand why Isolation alone is not enough

   Isolation prevents interference but does not guarantee correctness or crash safety alone.

3. Step 3: Combine properties for full guarantee

   Atomicity + Consistency + Durability together ensure safe, correct, and permanent transfers.

4. Final Answer:

   Atomicity, Consistency, and Durability -> Option A

5. Quick Check:

   Safe + correct + permanent = Atomicity + Consistency + Durability [OK]

Hint: All but Isolation ensure correctness and crash safety [OK]

Common Mistakes:

• Ignoring Consistency in correctness
• Thinking Isolation alone is enough
• Missing Durability for crash safety