Revoking privileges in MySQL - Time & Space Complexity

Time Complexity: Revoking privileges

O(n)

Understanding Time Complexity

When we revoke privileges in a database, the system checks and updates user permissions.

We want to understand how the time it takes changes as the number of users or privileges grows.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

REVOKE SELECT, INSERT ON database_name.* FROM 'user'@'host';

This command removes specific privileges from a user on all tables in a database.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The database system checks the privilege tables to find and update the user's privileges.
• How many times: It scans the privilege entries related to the user and the specified privileges once.

How Execution Grows With Input

The time to revoke privileges grows with the number of privilege entries the system must check.

Input Size (n)	Approx. Operations
10 privilege entries	10 checks
100 privilege entries	100 checks
1000 privilege entries	1000 checks

Pattern observation: The work increases directly with the number of privilege entries to check.

Final Time Complexity

Time Complexity: O(n)

This means the time to revoke privileges grows linearly with the number of privilege entries involved.

Common Mistake

[X] Wrong: "Revoking privileges happens instantly no matter how many users or privileges exist."

[OK] Correct: The system must find and update privilege records, so more entries mean more work and more time.

Interview Connect

Understanding how privilege changes scale helps you manage database security efficiently and shows you grasp how databases handle user permissions behind the scenes.

Self-Check

"What if we revoke privileges from multiple users at once? How would the time complexity change?"

Practice

1. What does the REVOKE statement do in MySQL?

easy

A. It removes specific privileges from a user.

B. It grants new privileges to a user.

C. It creates a new user in the database.

D. It deletes a database table.

Solution

1. Step 1: Understand the purpose of REVOKE

   The REVOKE statement is used to remove privileges that were previously granted to a user.

2. Step 2: Compare with other statements

   GRANT adds privileges, CREATE USER creates users, and DROP TABLE deletes tables, so they do not remove privileges.

3. Final Answer:

   It removes specific privileges from a user. -> Option A

4. Quick Check:

   REVOKE = Remove privileges [OK]

Hint: REVOKE always removes privileges, not adds [OK]

Common Mistakes:

• Confusing REVOKE with GRANT
• Thinking REVOKE creates users
• Assuming REVOKE deletes tables

2. Which of the following is the correct syntax to revoke SELECT privilege on the employees table from user 'john'@'localhost'?

easy

A. REVOKE SELECT ON database.employees FROM 'john'@'localhost';

B. REVOKE SELECT ON database.employees.* FROM 'john'@'localhost';

C. REVOKE SELECT ON employees.* FROM 'john'@'localhost';

D. REVOKE SELECT ON employees FROM 'john'@'localhost';

Solution

1. Step 1: Understand the syntax for REVOKE on a table

   The correct syntax requires specifying the database and table as database.table.

2. Step 2: Analyze each option

   REVOKE SELECT ON database.employees FROM 'john'@'localhost'; correctly uses database.employees format. Options A and C miss the database name, and B incorrectly adds .* after table name.

3. Final Answer:

   REVOKE SELECT ON database.employees FROM 'john'@'localhost'; -> Option A

4. Quick Check:

   REVOKE syntax needs database.table [OK]

Hint: Always specify database.table in REVOKE [OK]

Common Mistakes:

• Omitting database name
• Using .* after table name incorrectly
• Wrong user specification format

3. Given the following commands executed in MySQL:

GRANT SELECT, INSERT ON sales.orders TO 'anna'@'localhost';
REVOKE INSERT ON sales.orders FROM 'anna'@'localhost';
SHOW GRANTS FOR 'anna'@'localhost';

What privileges will 'anna'@'localhost' have on sales.orders after these commands?

medium

A. No privileges

B. SELECT and INSERT privileges

C. Only SELECT privilege

D. Only INSERT privilege

Solution

1. Step 1: Analyze the GRANT statement

   The user 'anna' is granted SELECT and INSERT privileges on sales.orders.

2. Step 2: Analyze the REVOKE statement

   The INSERT privilege is revoked from 'anna' on sales.orders, leaving only SELECT.

3. Final Answer:

   Only SELECT privilege -> Option C

4. Quick Check:

   REVOKE removes INSERT, SELECT remains [OK]

Hint: REVOKE removes specified privileges only [OK]

Common Mistakes:

• Assuming REVOKE removes all privileges
• Confusing which privilege was revoked
• Ignoring SHOW GRANTS output

4. Identify the error in this REVOKE statement:

REVOKE UPDATE ON mydb.* FROM 'mike'@'%';

medium

A. The user specification is incorrect.

B. The privilege name UPDATE is invalid.

C. The ON clause should specify a table, not a database.

D. The syntax is correct; no error.

Solution

1. Step 1: Check user specification

   'mike'@'%' is a valid user format allowing any host.

2. Step 2: Validate privilege and ON clause

   UPDATE is a valid privilege, and ON mydb.* means all tables in database mydb, which is allowed.

3. Final Answer:

   The syntax is correct; no error. -> Option D

4. Quick Check:

   REVOKE syntax with database.* is valid [OK]

Hint: ON database.* is valid for all tables [OK]

Common Mistakes:

• Thinking ON must specify a single table
• Confusing user host wildcard syntax
• Assuming UPDATE is invalid privilege

5. You want to revoke all privileges from user 'lisa'@'localhost' on the finance database but keep her SELECT privilege on the finance.reports table. Which sequence of commands achieves this?

hard

A. REVOKE SELECT ON finance.reports FROM 'lisa'@'localhost';REVOKE ALL PRIVILEGES ON finance.* FROM 'lisa'@'localhost';

B. REVOKE ALL PRIVILEGES ON finance.* FROM 'lisa'@'localhost';GRANT SELECT ON finance.reports TO 'lisa'@'localhost';

C. REVOKE ALL PRIVILEGES ON finance.reports FROM 'lisa'@'localhost';GRANT SELECT ON finance.* TO 'lisa'@'localhost';

D. REVOKE ALL PRIVILEGES ON finance.reports FROM 'lisa'@'localhost';REVOKE ALL PRIVILEGES ON finance.* FROM 'lisa'@'localhost';

Solution

1. Step 1: Revoke all privileges on the entire finance database

   Use REVOKE ALL PRIVILEGES ON finance.* to remove all rights on all tables in finance.

2. Step 2: Grant back SELECT privilege on finance.reports table

   Use GRANT SELECT ON finance.reports to restore only SELECT on that specific table.

3. Final Answer:

   REVOKE ALL PRIVILEGES ON finance.* FROM 'lisa'@'localhost'; GRANT SELECT ON finance.reports TO 'lisa'@'localhost'; -> Option B

4. Quick Check:

   Revoke all then grant SELECT on reports [OK]

Hint: Revoke all then grant needed privilege back [OK]

Common Mistakes:

• Revoking SELECT first removes needed privilege
• Granting SELECT on whole database instead of table
• Revoking privileges in wrong order