Joins Overview

So far, we have worked with data stored in a single table. In real-world databases, data is usually spread across multiple related tables.

SQL JOINs allow us to combine data from multiple tables and view it as a single result.

Why Do We Need Joins?

Databases are designed to avoid duplication. Instead of storing all information in one table, data is split into logical tables.

For example:

• Employee details in one table
• Department details in another table
• Project information in a separate table

To answer real questions, we must join these tables.

Example Tables

Consider the following two tables:

employees

+----+----------+---------------+--------+
| id | name     | department_id | salary |
+----+----------+---------------+--------+
| 1  | Alice    | 101           | 75000  |
| 2  | Bob      | 102           | 72000  |
| 3  | Charlie  | 101           | 60000  |
| 4  | Diana    | 103           | 80000  |
+----+----------+---------------+--------+
  

departments

+---------------+----------------+
| department_id | department_name|
+---------------+----------------+
| 101           | IT             |
| 102           | HR             |
| 103           | Finance        |
+---------------+----------------+
  

What is a JOIN?

A JOIN combines rows from two or more tables based on a related column.

In our example:

• employees.department_id
• departments.department_id

These columns create a relationship between the tables.

Basic JOIN Syntax

All joins follow a similar structure:

SELECT columns
FROM table1
JOIN table2
ON table1.column = table2.column;
  

The ON condition defines how rows are matched.

Types of SQL Joins

We will study each join type in detail in the next lessons.

Visual Understanding (Conceptual)

Think of joins like matching circles:

• INNER JOIN → only overlapping part
• LEFT JOIN → entire left circle + overlap
• RIGHT JOIN → entire right circle + overlap
• FULL JOIN → both circles entirely

Simple Join Example

Let’s join employees with their department names.

SELECT e.name, d.department_name
FROM employees e
INNER JOIN departments d
ON e.department_id = d.department_id;
  

Explanation:

• Tables are joined using department_id
• Each employee gets their department name
• Only matching records are returned

Table Aliases (Important)

Table aliases make queries easier to read.

• employees e
• departments d

Aliases are especially useful in joins and complex queries.

Common Beginner Mistakes

• Forgetting the ON condition
• Joining tables without related columns
• Confusing LEFT and RIGHT joins
• Not using table aliases

Why Joins Are Critical

Joins allow you to:

• Work with normalized databases
• Combine related information
• Build real-world reports
• Answer complex business questions

What’s Next?

In the next lesson, we will deep dive into INNER JOIN and understand how matching works in detail.