What are JOINs in SELECT?

A JOIN is used inside a SELECT query to combine data from two or more related tables.

Instead of storing everything in one large table, databases usually split related information into separate tables. A JOIN helps bring that related information back together when you need to read it.

A basic join pattern looks like this.

SELECT
    ...
FROM Employees AS e
JOIN Departments AS d
    ON e.department_id = d.department_id;

In SQL Server, JOIN by itself means INNER JOIN. It returns rows where the join condition matches in both tables.

Why JOINs are needed

Imagine you have two separate tables.

The Employees table stores employee details. The Departments table stores department details such as the department name and location.

To show each employee with their department location, SQL needs to connect both tables using a matching column.

Sample tables

We will join the Employees and Departments tables using department_id.

Joining two tables

The ON condition tells SQL how the two tables are related.

In this example, Employees.department_id is matched with Departments.department_id.

SELECT
    e.employee_name,
    d.department_name,
    d.location
FROM Employees AS e
INNER JOIN Departments AS d
    ON e.department_id = d.department_id;

Output after JOIN

The result can now show employee information together with department information.

Key takeaway