By the end of this lesson, you will be able to:

  • Explain what a relational database is

  • Describe how data is structured into tables, rows, and columns

  • Understand the purpose of primary keys (PK) and foreign keys (FK)

  • Interpret and handle NULL values in a relational context

What Is a Relational Database?

A relational database stores data in a structured way using tables. Each table represents a specific type of entity—like users, orders, or products.

  • Rows (also called records) represent individual data entries

  • Columns define the attributes of that data (like name, email, price)

Tables can be connected through keys, allowing you to link related data across the database. That’s where the “relational” part comes in.

Tables, Rows, and Columns

Here’s an example of a simple consumers table:

consumer_id (PK)nameemail
1Alicealice@email.com
2Bobbob@email.com

  • id, name, and email are columns

  • Each line is a row

  • The combination of column names and types is the schema

Primary Keys (PK)

A primary key is used to uniquely identify each row in a table. It must be:

  • Unique: no two rows can have the same value

  • Not null: every row must have a value

Example: 

CREATE TABLE users (
  user_id SERIAL PRIMARY KEY,
  name TEXT,
  email TEXT
);

Here, user_id is the primary key—each user gets a unique ID.

Foreign Keys (FK)

A foreign key is a column in one table that refers to the primary key of another table. It:

  • Creates a relationship between the two tables

  • Enforces referential integrity (you can’t reference a row that doesn’t exist)

Example:

CREATE TABLE orders (
  order_id SERIAL PRIMARY KEY,
  user_id INTEGER REFERENCES users(user_id),
  total DECIMAL
);

In this case, user_id in the orders table is a foreign key pointing to user_id in the users table.

Understanding NULL

A NULL value means “no value” or “unknown.” It’s different from zero or an empty string.

  • NULL does not equal anything—not even another NULL

  • Use IS NULL or IS NOT NULL in queries to filter them

SELECT * FROM users WHERE email IS NULL;

This query returns users who haven’t provided an email address.

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