SQL Inner Join

An INNER JOIN in SQL selects records that have matching values in both tables. If a row in one table does not have a corresponding match in the other, it is completely excluded from the result set.

By default, simply writing JOIN in your query performs an INNER JOIN.

Syntax

To implement an inner join, name your primary table, use the INNER JOIN keyword to declare the secondary table, and use the ON keyword to specify the connecting columns (usually a Primary Key matching a Foreign Key).

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

INNER JOIN: Identifies the second table you want to combine with your first.
ON: Specifies the matching condition (typically linking a primary key from one table to a foreign key in another).

Note: The word INNER is optional in almost all modern databases. Writing JOIN by itself defaults to an inner join.

Visual Concept & Practical Example

Think of an INNER JOIN like a Venn diagram intersection: only the overlapping data between the two sets is returned.

Sample Tables:

Customers Table

customer_id	name
1	Alice
2	Bob
3	Charlie

Orders Table

order_id	customer_id	product
101	1	Laptop
102	2	Phone
103	5	Tablet

The Query:

SELECT Customers.name, Orders.product
FROM Customers
INNER JOIN Orders
ON Customers.customer_id = Orders.customer_id;

Result:

name	product
Alice	Laptop
Bob	Phone

Why Charlie is missing: Charlie (customer_id 3) hasn't placed any orders, so there is no match in the Orders table.
Why Order 103 is missing: Order 103 belongs to customer_id 5, which doesn't exist in the Customers table.

Pro-Tip: Table Aliases

To avoid typing out long table names repeatedly and fix ambiguity errors, use short text aliases:

SELECT c.name, o.product
FROM Customers c
INNER JOIN Orders o 
ON c.customer_id = o.customer_id;

Combining INNER JOIN with a WHERE Filter

You can seamlessly add standard filtering rules to your joined datasets. The WHERE clause must always be placed after the join declarations.

-- Finds only the employees working in the Engineering department
SELECT e.name, d.department_name
FROM employees AS e
INNER JOIN departments AS d ON e.department_id = d.department_id
WHERE d.department_name = 'Engineering';

Joining Multiple Tables Seamlessly

If you need to string three or more tables together, you can simply chain subsequent INNER JOIN statements sequentially.

-- Connects a customer to their order, and that order to the specific product item
SELECT c.customer_name, o.order_date, p.product_name
FROM customers AS c
INNER JOIN orders AS o ON c.customer_id = o.customer_id
INNER JOIN order_items AS oi ON o.order_id = oi.order_id
INNER JOIN products AS p ON oi.product_id = p.product_id;