ACID properties in MySQL - 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 Consistency in ACID properties.

Consistency means a transaction must bring the database from one valid state to another, following all rules like constraints and triggers.

intermediate

What does Isolation ensure in database transactions?

Isolation ensures that concurrent transactions do not interfere with each other. Each transaction behaves as if it runs alone.

beginner

Define Durability in the context of ACID.

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

beginner

Why are ACID properties important in databases?

ACID properties keep data accurate and reliable by ensuring transactions are complete, consistent, isolated, and permanent.

Which ACID property guarantees that a transaction is fully completed or not done at all?

A Isolation

B Consistency

C Atomicity

D Durability

Atomicity means the transaction is all or nothing.

Which property ensures that database rules are not broken after a transaction?

A Consistency

B Atomicity

C Isolation

D Durability

Consistency ensures the database stays valid after transactions.

What does Isolation prevent in concurrent transactions?

A Data loss

B System crashes

C Slow queries

D Interference between transactions

Isolation keeps transactions from affecting each other.

Durability means that after a transaction commits, its changes are:

A Lost on crash

B Permanent

C Temporary

D Reversible

Durability guarantees changes stay even after crashes.

Which ACID property is about making sure a transaction behaves as if it runs alone?

A Isolation

B Atomicity

C Durability

D Consistency

Isolation means transactions do not see each other's partial changes.

Describe all four ACID properties and why each is important for database transactions.

Think about what happens if a transaction partially completes or if multiple transactions run at the same time.

Give a real-life example that illustrates the need for ACID properties in a banking system.

Imagine sending money from one account to another.

Practice

1. Which of the following is NOT one of the ACID properties in databases?

easy

A. Durability and Speed

B. Consistency

C. Isolation

D. Atomicity

Solution

1. Step 1: Recall the ACID properties

   ACID stands for Atomicity, Consistency, Isolation, and Durability.

2. Step 2: Identify the incorrect option

   Durability and Speed adds "Speed" which is not part of ACID properties.

3. Final Answer:

   Durability and Speed -> Option A

4. Quick Check:

   ACID excludes Speed [OK]

Hint: Remember ACID = Atomicity, Consistency, Isolation, Durability [OK]

Common Mistakes:

• Confusing performance terms with ACID properties
• Thinking Speed is part of ACID
• Mixing up Isolation with Integrity

2. Which SQL statement correctly starts a transaction in MySQL?

easy

A. BEGIN TRANSACTION;

B. START TRANSACTION;

C. OPEN TRANSACTION;

D. INIT TRANSACTION;

Solution

1. Step 1: Recall MySQL transaction syntax

   MySQL uses START TRANSACTION; to begin a transaction.

2. Step 2: Check other options

   Options A, B, and D are not valid MySQL commands for starting transactions.

3. Final Answer:

   START TRANSACTION; -> Option B

4. Quick Check:

   MySQL uses START TRANSACTION [OK]

Hint: Use START TRANSACTION to begin in MySQL [OK]

Common Mistakes:

• Using BEGIN TRANSACTION which is not valid in MySQL
• Confusing with other SQL dialects
• Typing incorrect keywords like OPEN or INIT

3. Consider this sequence in MySQL:

START TRANSACTION;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
UPDATE accounts SET balance = balance + 100 WHERE id = 2;
COMMIT;

What ACID property does this sequence mainly demonstrate?

medium

A. Consistency - data types are checked

B. Isolation - changes are visible immediately

C. Durability - changes are temporary

D. Atomicity - all changes happen or none

Solution

1. Step 1: Understand the transaction sequence

   The two updates are grouped inside a transaction with COMMIT at the end.

2. Step 2: Identify the ACID property

   Atomicity ensures either both updates happen or none do, preventing partial changes.

3. Final Answer:

   Atomicity - all changes happen or none -> Option D

4. Quick Check:

   Transaction groups changes atomically [OK]

Hint: Atomicity means all or nothing in transactions [OK]

Common Mistakes:

• Confusing Isolation with Atomicity
• Thinking Durability means temporary changes
• Assuming Consistency only checks data types

4. You wrote this MySQL transaction:

START TRANSACTION;
UPDATE inventory SET quantity = quantity - 5 WHERE product_id = 10;
COMMIT;

But sometimes the quantity becomes negative. What ACID property is likely violated?

medium

A. Consistency

B. Isolation

C. Durability

D. Atomicity

Solution

1. Step 1: Analyze the problem

   Quantity becoming negative means data rules are broken.

2. Step 2: Identify violated ACID property

   Consistency ensures data stays valid and follows rules; here it is violated.

3. Final Answer:

   Consistency -> Option A

4. Quick Check:

   Consistency keeps data valid [OK]

Hint: Consistency means data must follow rules always [OK]

Common Mistakes:

• Thinking Atomicity prevents negative values
• Confusing Isolation with data validity
• Assuming Durability fixes data rules

5. In a multi-user MySQL environment, two transactions run:

Transaction 1:
START TRANSACTION;
UPDATE accounts SET balance = balance - 50 WHERE id = 1;
-- waits before COMMIT

Transaction 2:
START TRANSACTION;
SELECT balance FROM accounts WHERE id = 1;
COMMIT;

Which ACID property ensures Transaction 2 does not see the uncommitted balance change from Transaction 1?

hard

A. Atomicity

B. Durability

C. Isolation

D. Consistency

Solution

1. Step 1: Understand concurrent transactions

   Transaction 1 updates but delays commit; Transaction 2 reads the same data.

2. Step 2: Identify ACID property controlling visibility

   Isolation ensures uncommitted changes are hidden from other transactions.

3. Final Answer:

   Isolation -> Option C

4. Quick Check:

   Isolation hides uncommitted changes [OK]

Hint: Isolation hides uncommitted changes from others [OK]

Common Mistakes:

• Confusing Isolation with Atomicity
• Thinking Durability controls visibility
• Assuming Consistency hides uncommitted data