Select policies for read access in Supabase - Mini Project: Build & Apply

Select policies for read access

📖 Scenario: You are managing a database table in Supabase that stores user profiles. You want to control who can read the data by creating policies that allow only certain users to access the profiles.

🎯 Goal: Create a read access policy for the profiles table that allows users to select (read) only their own profile data.

📋 What You'll Learn

Create a policy named Allow read own profile on the profiles table

The policy should allow SELECT operations

The policy condition must check that the auth.uid() matches the profiles.user_id

Enable the policy so it is active

💡 Why This Matters

🌍 Real World

Controlling who can read data in a database is essential for privacy and security in real applications like user profiles, orders, or messages.

💼 Career

Understanding how to write and enable row level security policies is a key skill for backend developers and cloud engineers working with managed databases like Supabase or PostgreSQL.

1

Create the profiles table

Create a table called profiles with columns user_id (UUID) and full_name (text).

Supabase

-- Your SQL code here to create the profiles table

Hint

Use CREATE TABLE statement with the specified columns and types.

2

Create a read policy for own profile

Create a policy named Allow read own profile on the profiles table that allows SELECT operations where auth.uid() equals profiles.user_id.

Supabase

-- Your SQL code here to create the read policy

Hint

Use CREATE POLICY with FOR SELECT and a USING clause that compares auth.uid() to user_id.

3

Enable the read policy

Enable the policy named Allow read own profile on the profiles table.

Supabase

-- Your SQL code here to enable the policy

Hint

Use ALTER TABLE to enable row level security and ALTER POLICY to enable the policy.

4

Test the read access policy

Write a SELECT statement to get all columns from profiles where user_id equals auth.uid(). This simulates a user reading their own profile.

Supabase

-- Your SQL code here to select own profile

Hint

Use SELECT * FROM profiles WHERE user_id = auth.uid() to test the policy.

Practice

1. What is the main purpose of a SELECT policy in Supabase?

easy

A. To control who can read data from a table

B. To control who can write data to a table

C. To create new tables in the database

D. To delete data from a table

Solution

1. Step 1: Understand the role of SELECT policies

   SELECT policies define rules for reading data from tables.

2. Step 2: Differentiate from other policies

   Write policies control data insertion or updates, not reading.

3. Final Answer:

   To control who can read data from a table -> Option A

4. Quick Check:

   SELECT policy = read access control [OK]

Hint: SELECT policies = rules for reading data only [OK]

Common Mistakes:

• Confusing SELECT with INSERT or UPDATE policies
• Thinking SELECT policies create or delete tables
• Assuming SELECT policies control write access

2. Which of the following is the correct syntax to create a read policy allowing all users to select from a table named profiles?

easy

A. CREATE POLICY "Allow all" ON profiles FOR UPDATE USING (true);

B. CREATE POLICY "Allow all" ON profiles FOR INSERT USING (true);

C. CREATE POLICY "Allow all" ON profiles FOR SELECT USING (true);

D. CREATE POLICY "Allow all" ON profiles FOR DELETE USING (true);

Solution

1. Step 1: Identify the correct command for read access

   FOR SELECT is used to allow read (select) operations.

2. Step 2: Confirm the condition for allowing all users

   USING (true) means no restriction, so all users can read.

3. Final Answer:

   CREATE POLICY "Allow all" ON profiles FOR SELECT USING (true); -> Option C

4. Quick Check:

   FOR SELECT + USING (true) = allow all read [OK]

Hint: FOR SELECT + USING (true) allows all reads [OK]

Common Mistakes:

• Using FOR INSERT instead of FOR SELECT
• Forgetting the USING clause
• Using FOR DELETE or UPDATE for read access

3. Given the policy:

CREATE POLICY "User can read own profile" ON profiles FOR SELECT USING (auth.uid() = user_id);

What will happen if a user tries to select a profile where user_id is different from their auth.uid()?

medium

A. The user will be able to read the profile

B. The user will get an empty result (no rows)

C. The query will cause a syntax error

D. The user will be able to update the profile

Solution

1. Step 1: Understand the policy condition

   The policy allows reading only if auth.uid() equals user_id.

2. Step 2: Analyze the case when user_id differs

   If user_id is different, the condition fails, so no rows are returned.

3. Final Answer:

   The user will get an empty result (no rows) -> Option B

4. Quick Check:

   Condition false = no rows returned [OK]

Hint: Read allowed only if auth.uid() matches user_id [OK]

Common Mistakes:

• Assuming user can read all profiles
• Expecting an error instead of empty result
• Confusing read with update permissions

4. You wrote this policy:

CREATE POLICY "Allow read" ON orders FOR SELECT USING (user_id = auth.uid);

But users cannot read their orders. What is the problem?

medium

A. USING clause should be omitted

B. The policy should use FOR UPDATE instead of FOR SELECT

C. The table name is incorrect

D. auth.uid should be called as a function: auth.uid()

Solution

1. Step 1: Check the usage of auth.uid

   auth.uid is a function and must be called with parentheses: auth.uid()

2. Step 2: Understand impact of missing parentheses

   Without (), the condition is invalid and blocks access.

3. Final Answer:

   auth.uid should be called as a function: auth.uid() -> Option D

4. Quick Check:

   auth.uid() is required for correct condition [OK]

Hint: Always call auth.uid() with parentheses [OK]

Common Mistakes:

• Using auth.uid without parentheses
• Confusing FOR SELECT with FOR UPDATE
• Removing USING clause mistakenly

5. You want to allow users to read only their own messages from a messages table. The table has columns id, content, and owner_id. Which policy correctly enforces this read access?

hard

A. CREATE POLICY "Read own messages" ON messages FOR SELECT USING (owner_id = auth.uid());

B. CREATE POLICY "Read own messages" ON messages FOR SELECT USING (owner_id != auth.uid());

C. CREATE POLICY "Read own messages" ON messages FOR SELECT USING (true);

D. CREATE POLICY "Read own messages" ON messages FOR UPDATE USING (owner_id = auth.uid());

Solution

1. Step 1: Identify correct condition for own data

   Users should read rows where owner_id equals their auth.uid()

2. Step 2: Confirm correct policy type and condition

   FOR SELECT with USING (owner_id = auth.uid()) restricts reads properly.

3. Final Answer:

   CREATE POLICY "Read own messages" ON messages FOR SELECT USING (owner_id = auth.uid()); -> Option A

4. Quick Check:

   owner_id = auth.uid() restricts read to own rows [OK]

Hint: Use owner_id = auth.uid() in USING for own read access [OK]

Common Mistakes:

• Using != instead of = in condition
• Allowing all reads with USING (true)
• Using FOR UPDATE instead of FOR SELECT