Lesson 7: Table Joins

35 min read Intermediate Lesson 7 of 12 12.4k learners

What are Joins?

In relational databases, data is often spread across multiple tables. A JOIN combines rows from two or more tables based on a related column.

INNER JOIN

Returns rows that have matching values in both tables.

SQL

SELECT c.first_name, c.last_name, o.order_id
FROM customers c
INNER JOIN orders o
    ON c.customer_id = o.customer_id;

LEFT JOIN

Returns all rows from the left table, and matching rows from the right table. Unmatched rows show NULL values.

SQL

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

RIGHT JOIN

Returns all rows from the right table, and matching rows from the left table.

SQL

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

FULL JOIN

Returns all rows from both tables, matching when possible, and filling NULLs when there’s no match. (Note: not supported in MySQL, but available in PostgreSQL, SQL Server, Oracle).

SQL

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