Lock types (shared, exclusive) in MySQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to acquire a shared lock on the table.

MySQL

LOCK TABLES my_table [1];

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

AWRITE

BEXCLUSIVE

CREAD

DSHARED

Attempts:

3 left

💡 Hint

Common Mistakes

Using WRITE instead of READ for shared locks.

Using EXCLUSIVE or SHARED keywords which are not valid in MySQL LOCK TABLES.

In MySQL, READ lock is a shared lock allowing concurrent reads.

2fill in blank

medium

Complete the code to acquire an exclusive lock on the table.

MySQL

LOCK TABLES my_table [1];

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

AREAD

BWRITE

CSHARED

DEXCLUSIVE

Attempts:

3 left

💡 Hint

Common Mistakes

Using READ instead of WRITE for exclusive locks.

Using SHARED or EXCLUSIVE keywords which are not valid in MySQL LOCK TABLES.

In MySQL, WRITE lock is an exclusive lock that prevents other sessions from reading or writing.

3fill in blank

hard

Fix the error in the code to properly release all locks.

MySQL

[1];

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

AUNLOCK TABLES

BRELEASE LOCKS

CCOMMIT

DROLLBACK

Attempts:

3 left

💡 Hint

Common Mistakes

Using RELEASE LOCKS which is not a valid MySQL command.

Using COMMIT or ROLLBACK which do not release table locks.

To release all table locks in MySQL, use UNLOCK TABLES.

4fill in blank

hard

Fill both blanks to lock the table for reading and then unlock it.

MySQL

LOCK TABLES my_table [1];
-- do some reading
[2];

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

AREAD

BUNLOCK TABLES

CWRITE

DCOMMIT

Attempts:

3 left

💡 Hint

Common Mistakes

Using WRITE instead of READ for shared lock.

Using COMMIT instead of UNLOCK TABLES to release locks.

First, acquire a shared (READ) lock, then release all locks with UNLOCK TABLES.

5fill in blank

hard

Fill all three blanks to lock the table exclusively, perform an update, and then unlock.

MySQL

LOCK TABLES my_table [1];
UPDATE my_table SET col = 1 WHERE id = 5;
[2];

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

AREAD

BUNLOCK TABLES

CWRITE

DCOMMIT

Attempts:

3 left

💡 Hint

Common Mistakes

Using READ lock when updating data.

Using COMMIT instead of UNLOCK TABLES to release locks.

Use WRITE lock for exclusive access, then UNLOCK TABLES to release. The third blank is unused in code but included to meet schema.

Practice

1. What is the main purpose of a shared lock in MySQL?

easy

A. Allow multiple users to read data simultaneously without conflicts

B. Prevent any user from reading data

C. Allow only one user to write data at a time

D. Delete data safely from the database

Solution

1. Step 1: Understand shared lock behavior

   A shared lock allows many users to read the same data at the same time safely.

2. Step 2: Compare with other lock types

   Exclusive locks prevent others from reading or writing, but shared locks only prevent writing.

3. Final Answer:

   Allow multiple users to read data simultaneously without conflicts -> Option A

4. Quick Check:

   Shared lock = multiple readers allowed [OK]

Hint: Shared locks allow many readers, exclusive locks block others [OK]

Common Mistakes:

• Confusing shared lock with exclusive lock
• Thinking shared locks block reading
• Assuming shared locks allow writing

2. Which of the following is the correct syntax to acquire a shared lock on a table in MySQL?

easy

A. LOCK TABLES table_name WRITE;

B. LOCK TABLES table_name SHARE;

C. LOCK TABLES table_name EXCLUSIVE;

D. LOCK TABLES table_name READ;

Solution

1. Step 1: Recall MySQL lock syntax

   In MySQL, to acquire a shared lock, you use LOCK TABLES table_name READ;.

2. Step 2: Eliminate incorrect options

   WRITE is for exclusive locks, EXCLUSIVE and SHARE are not valid MySQL lock keywords.

3. Final Answer:

   LOCK TABLES table_name READ; -> Option D

4. Quick Check:

   Shared lock syntax uses READ keyword [OK]

Hint: Use READ for shared lock, WRITE for exclusive lock [OK]

Common Mistakes:

• Using WRITE instead of READ for shared lock
• Using unsupported keywords like EXCLUSIVE or SHARE
• Confusing lock syntax with SQL SELECT statements

3. Consider two transactions in MySQL: Transaction A acquires a shared lock on a row, and Transaction B tries to acquire an exclusive lock on the same row. What happens?

medium

A. Transaction B immediately acquires the exclusive lock

B. Transaction B waits until Transaction A releases the shared lock

C. Transaction A releases its shared lock automatically

D. Both transactions acquire locks simultaneously

Solution

1. Step 1: Understand lock compatibility

   Shared locks allow multiple readers but block exclusive locks until released.

2. Step 2: Analyze the scenario

   Transaction B must wait for Transaction A to release the shared lock before getting exclusive lock.

3. Final Answer:

   Transaction B waits until Transaction A releases the shared lock -> Option B

4. Quick Check:

   Exclusive lock waits for shared locks to release [OK]

Hint: Exclusive locks wait if shared locks exist on data [OK]

Common Mistakes:

• Assuming exclusive lock can override shared lock immediately
• Thinking shared lock releases automatically
• Believing both locks can coexist on same data

4. You wrote this MySQL code to lock a table for writing but it causes an error:

LOCK TABLES my_table SHARE;

What is the error and how to fix it?

medium

A. Error: Missing semicolon. Fix: Add semicolon at end.

B. Error: Table does not exist. Fix: Create the table first.

C. Error: Invalid lock type 'SHARE'. Fix: Use 'WRITE' instead of 'SHARE'.

D. Error: Cannot lock table twice. Fix: Unlock before locking again.

Solution

1. Step 1: Identify invalid lock keyword

   MySQL does not support 'SHARE' as a lock type; it uses 'READ' or 'WRITE'.

2. Step 2: Correct the lock type

   To lock for writing, use LOCK TABLES my_table WRITE; instead.

3. Final Answer:

   Error: Invalid lock type 'SHARE'. Fix: Use 'WRITE' instead of 'SHARE'. -> Option C

4. Quick Check:

   Use WRITE for exclusive lock, SHARE is invalid [OK]

Hint: Use READ or WRITE only; SHARE is not valid in MySQL [OK]

Common Mistakes:

• Using unsupported lock keywords
• Ignoring error messages about syntax
• Assuming SHARE is synonym for READ

5. You want to ensure that multiple users can read data from a table simultaneously, but only one user can update it at a time. Which locking strategy should you use in MySQL?

hard

A. Use shared locks (READ) for reading and exclusive locks (WRITE) for updating

B. Use exclusive locks (WRITE) for both reading and updating

C. Use shared locks (READ) for updating and exclusive locks (WRITE) for reading

D. Do not use any locks; rely on automatic commit

Solution

1. Step 1: Understand shared and exclusive lock roles

   Shared locks allow multiple readers; exclusive locks allow one writer.

2. Step 2: Apply correct locks for reading and writing

   Use shared locks for reading so many users can read safely, and exclusive locks for updating to prevent conflicts.

3. Final Answer:

   Use shared locks (READ) for reading and exclusive locks (WRITE) for updating -> Option A

4. Quick Check:

   Shared for read, exclusive for write ensures safe concurrency [OK]

Hint: Shared locks for reading, exclusive locks for writing [OK]

Common Mistakes:

• Using exclusive locks for reading blocks all users
• Using shared locks for writing causes conflicts
• Ignoring locks and risking data corruption