SQL Joins Explained

Why Joins Matter

Relational databases store data across multiple related tables to avoid duplication. Joins let you combine rows from two or more tables based on a related column, such as a customer ID, so you can query them together.

Inner Join

An INNER JOIN returns only the rows that have matching values in both tables.

SELECT o.order_id, c.customer_name, o.order_amount
FROM orders o
INNER JOIN customers c
    ON o.customer_id = c.customer_id;

Left Join

A LEFT JOIN returns all rows from the left table, and matching rows from the right table. Unmatched right-side columns return NULL.

SELECT c.customer_name, o.order_id
FROM customers c
LEFT JOIN orders o
    ON c.customer_id = o.customer_id;

Right Join

A RIGHT JOIN is the mirror of a left join — it returns all rows from the right table, and matching rows from the left table.

SELECT c.customer_name, o.order_id
FROM customers c
RIGHT JOIN orders o
    ON c.customer_id = o.customer_id;

Full Join

A FULL JOIN returns all rows from both tables, matching where possible and filling unmatched sides with NULL.

SELECT c.customer_name, o.order_id
FROM customers c
FULL JOIN orders o
    ON c.customer_id = o.customer_id;