Table relationships in Supabase

Introduction

Table relationships help connect data in different tables so you can find related information easily.

You want to link customers to their orders in a shopping app.

You need to connect blog posts to their authors.

You want to track which products belong to which categories.

You want to see all comments related to a specific post.

You want to organize data so it is easy to update and query.

Syntax

Supabase

CREATE TABLE child_table (
  id SERIAL PRIMARY KEY,
  parent_id INTEGER REFERENCES parent_table(id)
);

The REFERENCES keyword creates a link from one table to another using a column.

The linked column usually is a primary key in the parent table.

Examples

This example links posts to authors using author_id.

Supabase

CREATE TABLE authors (
  id SERIAL PRIMARY KEY,
  name TEXT NOT NULL
);

CREATE TABLE posts (
  id SERIAL PRIMARY KEY,
  author_id INTEGER REFERENCES authors(id),
  title TEXT NOT NULL
);

Products are connected to categories by category_id.

Supabase

CREATE TABLE categories (
  id SERIAL PRIMARY KEY,
  name TEXT NOT NULL
);

CREATE TABLE products (
  id SERIAL PRIMARY KEY,
  category_id INTEGER REFERENCES categories(id),
  product_name TEXT NOT NULL
);

Sample Program

This creates two tables: customers and orders. Orders link to customers using customer_id. Then it inserts sample data and shows how to get orders with customer names.

Supabase

CREATE TABLE customers (
  id SERIAL PRIMARY KEY,
  name TEXT NOT NULL
);

CREATE TABLE orders (
  id SERIAL PRIMARY KEY,
  customer_id INTEGER REFERENCES customers(id),
  order_date DATE NOT NULL
);

-- Insert sample data
INSERT INTO customers (name) VALUES ('Alice'), ('Bob');
INSERT INTO orders (customer_id, order_date) VALUES (1, '2024-06-01'), (2, '2024-06-02');

-- Query orders with customer names
SELECT orders.id, customers.name, orders.order_date
FROM orders
JOIN customers ON orders.customer_id = customers.id;

Important Notes

Always use primary keys in parent tables for reliable links.

Use JOIN queries to get related data from connected tables.

Relationships help keep data organized and avoid duplicates.

Summary

Table relationships connect data across tables using keys.

They make it easy to find related information with JOINs.

Use REFERENCES to create these links safely and clearly.

Practice

1. What is the main purpose of creating table relationships in Supabase?

easy

A. To store files in the database

B. To connect data across tables using keys

C. To increase the size of the database

D. To change the database theme

Solution

1. Step 1: Understand table relationships

   Table relationships link data in different tables using keys to keep data connected.

2. Step 2: Identify the purpose

   This linking helps to find related data easily, like joining customer info with orders.

3. Final Answer:

   To connect data across tables using keys -> Option B

4. Quick Check:

   Table relationships = Connect data [OK]

Hint: Relationships link tables by keys for easy data access [OK]

Common Mistakes:

• Thinking relationships store files
• Believing relationships increase database size
• Confusing relationships with UI themes

2. Which SQL keyword is used in Supabase to create a foreign key relationship between tables?

easy

A. REFERENCES

B. JOIN

C. SELECT

D. INSERT

Solution

1. Step 1: Identify the keyword for foreign keys

   In SQL, the REFERENCES keyword defines a foreign key that links one table's column to another's primary key.

2. Step 2: Match with Supabase usage

   Supabase uses standard SQL, so REFERENCES is used to create safe links between tables.

3. Final Answer:

   REFERENCES -> Option A

4. Quick Check:

   Foreign key = REFERENCES [OK]

Hint: Use REFERENCES to link tables with foreign keys [OK]

Common Mistakes:

• Using JOIN to create relationships (JOIN reads data, doesn't create keys)
• Confusing SELECT or INSERT with key creation

3. Given these two tables:
CREATE TABLE authors (id SERIAL PRIMARY KEY, name TEXT);
CREATE TABLE books (id SERIAL PRIMARY KEY, title TEXT, author_id INT REFERENCES authors(id));
What will this query return?
SELECT books.title, authors.name FROM books JOIN authors ON books.author_id = authors.id;

medium

A. A list of books without author names

B. A list of authors without book titles

C. An error because of missing foreign key

D. A list of book titles with their author's names

Solution

1. Step 1: Understand the table relationship

   The books table has a foreign key author_id linking to authors.id, connecting each book to its author.

2. Step 2: Analyze the JOIN query

   The query joins books and authors on matching author_id and id, selecting book titles and author names together.

3. Final Answer:

   A list of book titles with their author's names -> Option D

4. Quick Check:

   JOIN on foreign key returns related data [OK]

Hint: JOIN returns combined rows matching foreign keys [OK]

Common Mistakes:

• Expecting authors without books from this JOIN
• Thinking query causes error due to foreign key
• Assuming books without author names appear

4. You wrote this table creation:
CREATE TABLE orders (id SERIAL PRIMARY KEY, customer_id INT, FOREIGN KEY customer_id REFERENCES customers(id));
Why does this cause an error in Supabase?

medium

A. FOREIGN KEY syntax is incorrect; it needs parentheses around the column

B. customer_id must be TEXT, not INT

C. PRIMARY KEY cannot be SERIAL

D. REFERENCES keyword is not supported

Solution

1. Step 1: Check FOREIGN KEY syntax

   The FOREIGN KEY clause requires parentheses around the column name, like FOREIGN KEY (customer_id).

2. Step 2: Confirm other parts

   customer_id as INT is valid, SERIAL primary key is valid, and REFERENCES is supported in Supabase.

3. Final Answer:

   FOREIGN KEY syntax is incorrect; it needs parentheses around the column -> Option A

4. Quick Check:

   FOREIGN KEY syntax = parentheses needed [OK]

Hint: Put column names in parentheses after FOREIGN KEY [OK]

Common Mistakes:

• Omitting parentheses around foreign key column
• Changing data type unnecessarily
• Thinking REFERENCES is unsupported

5. You want to create a many-to-many relationship between students and courses tables in Supabase. Which setup is best?

hard

A. Add a foreign key column in students pointing to courses

B. Add a foreign key column in courses pointing to students

C. Create a join table with student_id and course_id as foreign keys

D. Merge students and courses into one table

Solution

1. Step 1: Understand many-to-many relationships

   Many-to-many means each student can take many courses and each course can have many students.

2. Step 2: Choose the correct design

   Neither students nor courses alone can hold all links. A join table with student_id and course_id foreign keys connects them properly.

3. Final Answer:

   Create a join table with student_id and course_id as foreign keys -> Option C

4. Quick Check:

   Many-to-many = join table with two foreign keys [OK]

Hint: Use a join table for many-to-many links [OK]

Common Mistakes:

• Trying to store many-to-many in one table column
• Merging unrelated tables
• Using one foreign key only