Column-level permissions in PostgreSQL

Introduction

Column-level permissions let you control who can see or change specific columns in a table. This keeps sensitive data safe.

You want to hide salary information from most employees but allow HR to see it.

You want to let users update their contact info but not their user ID.

You want to show product names to customers but hide supplier details.

You want to restrict access to credit card numbers in a customer table.

You want to allow read-only access to some columns but full access to others.

Syntax

PostgreSQL

GRANT SELECT (column_name) ON table_name TO role_name;
REVOKE SELECT (column_name) ON table_name FROM role_name;

You specify the columns inside parentheses after the privilege type.

You can grant or revoke permissions on specific columns separately from the whole table.

Examples

This lets the sales team see only the name and email columns in the customers table.

PostgreSQL

GRANT SELECT (name, email) ON customers TO sales_team;

This removes permission for everyone to see the salary column in the employees table.

PostgreSQL

REVOKE SELECT (salary) ON employees FROM public;

This allows the support team to update only the phone column in the customers table.

PostgreSQL

GRANT UPDATE (phone) ON customers TO support_team;

Sample Program

This example creates an employees table and a role hr_team. It grants hr_team permission to see only the name and email columns, not salary. Then it switches to hr_team role and tries to select all columns.

PostgreSQL

CREATE TABLE employees (
  id SERIAL PRIMARY KEY,
  name TEXT,
  email TEXT,
  salary NUMERIC
);

INSERT INTO employees (name, email, salary) VALUES
('Alice', '[e-mail]', 70000),
('Bob', '[e-mail]', 60000);

CREATE ROLE hr_team;

GRANT SELECT (name, email) ON employees TO hr_team;
REVOKE SELECT (salary) ON employees FROM hr_team;

SET ROLE hr_team;

SELECT * FROM employees;

Important Notes

If a role does not have permission on a column, that column will show as NULL when selected.

Column-level permissions work together with table-level permissions. You need SELECT on the table or on the columns.

Summary

Column-level permissions control access to specific columns in a table.

Use GRANT and REVOKE with column names in parentheses.

Restricted columns show as NULL if the user lacks permission.

Practice

1. What does column-level permission in PostgreSQL control?

easy

A. Access to server configuration files

B. Access to entire tables only

C. Access to database schemas

D. Access to specific columns in a table

Solution

1. Step 1: Understand the concept of column-level permissions

   Column-level permissions allow control over which columns a user can see or modify in a table.

2. Step 2: Compare with other access types

   Other options refer to broader or unrelated access controls, not specific columns.

3. Final Answer:

   Access to specific columns in a table -> Option D

4. Quick Check:

   Column-level permission = Access to specific columns [OK]

Hint: Column-level means controlling access per column, not whole table [OK]

Common Mistakes:

• Confusing column-level with table-level permissions
• Thinking it controls schema or server access
• Assuming it controls row-level access

2. Which of the following is the correct syntax to grant SELECT permission on column email of table users to user alice?

easy

A. GRANT SELECT(email) ON users TO alice;

B. GRANT SELECT ON users TO alice(email);

C. GRANT SELECT ON users(email) TO alice;

D. GRANT SELECT TO alice ON users(email);

Solution

1. Step 1: Recall PostgreSQL syntax for column-level GRANT

   The correct syntax is GRANT SELECT (email) ON users TO alice; (parentheses after privilege).

2. Step 2: Match options with syntax

   GRANT SELECT(email) ON users TO alice; matches the correct syntax exactly, others misplace keywords or parentheses.

3. Final Answer:

   GRANT SELECT(email) ON users TO alice; -> Option A

4. Quick Check:

   GRANT SELECT(column) ON table TO user [OK]

Hint: GRANT SELECT(column) ON table TO user is correct syntax [OK]

Common Mistakes:

• Placing column name after TO user
• Putting column inside ON table()
• Misordering keywords in the statement

3. Given the table employees(id, name, salary), if user bob has SELECT permission only on id and name, what will be the result of SELECT * FROM employees; executed by bob?

medium

A. Only id and name columns with data, salary as NULL

B. All columns with actual data

C. Error: permission denied

D. Only salary column with data

Solution

1. Step 1: Understand column-level permission effect on SELECT *

   User can see columns they have permission for; restricted columns appear as NULL.

2. Step 2: Apply to given columns

   Bob has permission on id and name, so salary shows as NULL.

3. Final Answer:

   Only id and name columns with data, salary as NULL -> Option A

4. Quick Check:

   Restricted columns show NULL, not error [OK]

Hint: Restricted columns appear as NULL, not error [OK]

Common Mistakes:

• Expecting a permission denied error
• Assuming all columns show data
• Thinking restricted columns are hidden completely

4. You run the command GRANT SELECT ON employees(name, salary) TO carol; but get a syntax error. What is the likely cause?

medium

A. Column names must be in double quotes

B. Cannot grant permissions on multiple columns at once

C. Column list in parentheses must precede the table name

D. User carol does not exist

Solution

1. Step 1: Check syntax for GRANT with multiple columns

   Correct syntax is GRANT SELECT (name, salary) ON employees TO carol;

2. Step 2: Identify error in command

   Command incorrectly places column list after table name; column list must follow SELECT before ON.

3. Final Answer:

   Column list in parentheses must precede the table name -> Option C

4. Quick Check:

   GRANT SELECT (col1, col2) ON table TO user [OK]

Hint: Columns inside parentheses after SELECT before ON table [OK]

Common Mistakes:

• Placing column list after table name
• Forgetting parentheses around columns
• Assuming multiple columns need separate GRANTs

5. You want to allow user dave to update only the phone column in the contacts table but not others. Which sequence of commands correctly achieves this?

hard

A. GRANT UPDATE(phone) ON contacts TO dave; REVOKE UPDATE ON contacts FROM dave;

B. REVOKE UPDATE ON contacts FROM dave; GRANT UPDATE(phone) ON contacts TO dave;

C. GRANT UPDATE ON contacts TO dave; REVOKE UPDATE(phone) ON contacts FROM dave;

D. GRANT UPDATE(phone) ON contacts TO dave;

Solution

1. Step 1: Remove any existing full UPDATE permission

   First revoke any broad UPDATE permission to avoid conflicts.

2. Step 2: Grant UPDATE permission only on the phone column

   Then grant UPDATE on the specific column to limit access.

3. Final Answer:

   REVOKE UPDATE ON contacts FROM dave; GRANT UPDATE(phone) ON contacts TO dave; -> Option B

4. Quick Check:

   Revoke broad then grant column-level UPDATE [OK]

Hint: Revoke broad permission before granting column-level update [OK]

Common Mistakes:

• Granting column-level without revoking broad permission
• Revoking column-level instead of broad permission
• Assuming single GRANT is enough if broad permission exists