Table relationships in Supabase - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a table relationship in a database?

A table relationship connects two tables so they can share related data, like linking a list of orders to the customers who made them.

beginner

What is a foreign key in table relationships?

A foreign key is a column in one table that points to the primary key in another table, helping link related data between tables.

intermediate

Explain the difference between one-to-many and many-to-many relationships.

One-to-many means one record in a table relates to many records in another (like one customer has many orders). Many-to-many means records in both tables can relate to many records in the other (like students and classes).

beginner

How does Supabase help manage table relationships?

Supabase lets you define foreign keys and use its interface or SQL to create relationships, making it easy to connect tables and query related data.

beginner

Why are table relationships important in cloud databases?

They keep data organized and connected, reduce duplication, and make it easier to retrieve related information quickly and accurately.

What does a foreign key do in a table relationship?

A Links a record in one table to a record in another table

B Stores large files in the database

C Creates a backup of the table

D Deletes duplicate records automatically

A foreign key connects one table's record to another table's record, establishing a relationship.

Which relationship type means one record relates to many records in another table?

A One-to-one

B Many-to-many

C One-to-many

D No relationship

One-to-many means one record in a table connects to many records in another table.

In Supabase, how can you create a table relationship?

A By defining a foreign key between tables

B By uploading CSV files

C By changing the database password

D By deleting tables

Defining foreign keys is how you create relationships between tables in Supabase.

Why are table relationships useful?

A They remove all data duplication automatically

B They increase the size of the database

C They slow down queries

D They help organize and connect data efficiently

Table relationships organize data and connect related information for efficient use.

What is a many-to-many relationship example?

A One product belongs to one category

B Students enroll in many classes, and classes have many students

C One customer has many orders

D One employee has one ID

Many-to-many means both sides can have many related records, like students and classes.

Describe how foreign keys create relationships between tables in Supabase.

Think about how one table points to another.

Explain the difference between one-to-many and many-to-many relationships with examples.

Use simple real-life examples to compare.

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