Join

The JOIN (or INNER JOIN) keyword is used to combine rows from two or more tables based on a related column. By joining tables, you can retrieve data spread across multiple tables and analyze it as a single dataset.

1. Basic Syntax

The syntax for an INNER JOIN looks like this:

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

• table1 and table2 are the tables being joined.
• ON specifies the condition for matching rows between the two tables.

2. Example with Two Tables

Imagine you have two tables:
employees table:

id	name	department_id
1	Alice	101
2	Bob	102
3	Charlie	NULL

departments table:

id	department_name
101	HR
102	IT
103	Finance

Query:

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

Result:

name	department_name
Alice	HR
Bob	IT

• Only rows where employees.department_id matches departments.id are included.
• Charlie is excluded because department_id is NULL, and no match exists in the departments table.

3. Multiple Joins

You can use INNER JOIN to join more than two tables. For example:

SELECT e.name, d.department_name, p.project_name 
FROM employees e 
INNER JOIN departments d ON e.department_id = d.id 
INNER JOIN projects p ON e.id = p.employee_id;

• This joins three tables: employees, departments, and projects.

4. Aliases for Readability

Aliases make queries easier to read and write:

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