10.2 INNER JOIN

Level: 10 | Version: v1.1 | Author: Meptrasoft

Overview

INNER JOIN returns only the rows that have a match in both tables.

For example:

CUSTOMERS
    +
ORDERS
    ↓
INNER JOIN
    ↓
MATCHING RECORDS ONLY

INNER JOIN → Keep matching rows from both tables.

Practice Tables

This topic uses two tables:

Customers

Orders

Customers 8, 9, and 10 have no matching orders.

Basic INNER JOIN

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

Result:

Only customers with matching orders are returned.

How INNER JOIN Works

The database compares the join columns:

customers.customer_id
          =
orders.customer_id

For example:

Customer ID 1 → Order ID 1
Customer ID 1 → Order ID 3
Customer ID 2 → Order ID 2

A customer can therefore appear more than once when that customer has multiple orders.

One matching customer can produce multiple result rows.

Using Table Aliases

Aliases make join queries shorter and easier to read.

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

Here:

c → customers
o → orders

INNER JOIN vs No Match

Suppose customer_id = 8 has no order.

Customers → 8
Orders    → no 8

With INNER JOIN, customer 8 is not returned.

MATCH → INCLUDED
NO MATCH → EXCLUDED

This is the key difference to remember before learning LEFT JOIN.

Common Beginner Mistakes

Placement Quick Points

INNER JOIN
→ MATCHING ROWS ONLY

ON
→ JOIN CONDITION

MATCH
→ INCLUDED

NO MATCH
→ EXCLUDED

• INNER JOIN combines matching rows from two tables.
• The ON clause defines how the tables are matched.
• Unmatched rows are excluded.
• One row from one table can match multiple rows in the other table.
• Aliases make join queries easier to read.

Interview Questions

What is an INNER JOIN?

An INNER JOIN returns rows where the join condition matches in both tables.

What happens to unmatched rows?

They are excluded from the result.

Can one customer appear multiple times?

Yes. If the customer has multiple matching orders, the customer can appear in multiple result rows.

What is the purpose of ON?

ON specifies the condition used to match rows between the tables.

Does INNER JOIN modify the original tables?

No. It produces a query result without changing the source tables.

What is the difference between JOIN and INNER JOIN?

JOIN is commonly used as shorthand for INNER JOIN.

Practice & Hands-On Exercises

Using customers and orders:

1. Display customer names with their order IDs.
2. Display customer names with order amounts.
3. Find customers who have at least one order.
4. Find customers who placed multiple orders.
5. Explain why customers 8, 9, and 10 do not appear.
6. Rewrite the query using table aliases:

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

💡 Tip: Test your queries using the in-browser interactive runner above.

Key Takeaway

CUSTOMERS
    +
ORDERS
    ↓
INNER JOIN
    ↓
MATCHING RECORDS

INNER JOIN returns only the rows that match between the joined tables.

End of INNER JOIN