INNER JOIN syntax in SQL - Mini Project: Build & Apply

Using INNER JOIN to Combine Customer and Order Data

📖 Scenario: You work at a small online store. You have two tables: one with customer information and one with their orders. You want to see which customers placed which orders.

🎯 Goal: Build a SQL query using INNER JOIN to combine the customers and orders tables on the customer_id field.

📋 What You'll Learn

Create a customers table with customer_id and customer_name columns.

Create an orders table with order_id, customer_id, and order_date columns.

Write a SQL query using INNER JOIN to combine these tables on customer_id.

Select customer_name and order_date from the joined tables.

💡 Why This Matters

🌍 Real World

Combining customer and order data is common in sales and business reporting to understand customer activity.

💼 Career

Knowing how to write INNER JOIN queries is essential for database analysts, developers, and anyone working with relational databases.

1

Create the customers table

Write SQL code to create a table called customers with columns customer_id as an integer primary key and customer_name as text. Insert these exact rows: (1, 'Alice'), (2, 'Bob'), (3, 'Charlie').

SQL

-- Create the customers table and insert data
-- Your code here

Hint

Use CREATE TABLE to define the table and INSERT INTO to add rows.

2

Create the orders table

Write SQL code to create a table called orders with columns order_id as an integer primary key, customer_id as integer, and order_date as text. Insert these exact rows: (101, 1, '2024-01-10'), (102, 2, '2024-01-11'), (103, 1, '2024-01-12').

SQL

CREATE TABLE customers (
  customer_id INTEGER PRIMARY KEY,
  customer_name TEXT
);

INSERT INTO customers (customer_id, customer_name) VALUES
  (1, 'Alice'),
  (2, 'Bob'),
  (3, 'Charlie');

-- Create the orders table and insert data
-- Your code here

Hint

Remember to define order_id as the primary key and include customer_id to link to customers.

3

Write the INNER JOIN query

Write a SQL query that uses INNER JOIN to combine customers and orders on customer_id. Select customer_name and order_date from the joined tables.

SQL

CREATE TABLE customers (
  customer_id INTEGER PRIMARY KEY,
  customer_name TEXT
);

INSERT INTO customers (customer_id, customer_name) VALUES
  (1, 'Alice'),
  (2, 'Bob'),
  (3, 'Charlie');

CREATE TABLE orders (
  order_id INTEGER PRIMARY KEY,
  customer_id INTEGER,
  order_date TEXT
);

INSERT INTO orders (order_id, customer_id, order_date) VALUES
  (101, 1, '2024-01-10'),
  (102, 2, '2024-01-11'),
  (103, 1, '2024-01-12');

-- Write the INNER JOIN query below
-- Your code here

Hint

Use INNER JOIN with ON customers.customer_id = orders.customer_id to link the tables.

4

Complete the query with ordering

Add an ORDER BY clause to the previous query to sort the results by order_date in ascending order.

SQL

CREATE TABLE customers (
  customer_id INTEGER PRIMARY KEY,
  customer_name TEXT
);

INSERT INTO customers (customer_id, customer_name) VALUES
  (1, 'Alice'),
  (2, 'Bob'),
  (3, 'Charlie');

CREATE TABLE orders (
  order_id INTEGER PRIMARY KEY,
  customer_id INTEGER,
  order_date TEXT
);

INSERT INTO orders (order_id, customer_id, order_date) VALUES
  (101, 1, '2024-01-10'),
  (102, 2, '2024-01-11'),
  (103, 1, '2024-01-12');

SELECT customers.customer_name, orders.order_date
FROM customers
INNER JOIN orders ON customers.customer_id = orders.customer_id
-- Add ORDER BY clause below
-- Your code here

Hint

Use ORDER BY orders.order_date ASC to sort the results by date.

Practice

1. What does an INNER JOIN do in SQL?

easy

A. It returns all rows from the first table only.

B. It returns rows that have matching values in both tables.

C. It returns all rows from the second table only.

D. It returns all rows from both tables, matching or not.

Solution

1. Step 1: Understand the purpose of INNER JOIN

   INNER JOIN combines rows from two tables where the join condition matches in both tables.

2. Step 2: Compare options with INNER JOIN behavior

   Only the description "It returns rows that have matching values in both tables." correctly states that it returns rows with matching values in both tables.

3. Final Answer:

   It returns rows that have matching values in both tables. -> Option B

4. Quick Check:

   INNER JOIN = matching rows only [OK]

Hint: INNER JOIN returns only matching rows from both tables [OK]

Common Mistakes:

• Thinking INNER JOIN returns all rows from one table
• Confusing INNER JOIN with LEFT or RIGHT JOIN
• Assuming it returns unmatched rows

2. Which of the following is the correct syntax for an INNER JOIN between tables Employees and Departments on the column DeptID using the ON clause?

easy

A. SELECT * FROM Employees INNER JOIN Departments ON Employees.DeptID == Departments.DeptID;

B. SELECT * FROM Employees JOIN Departments WHERE Employees.DeptID = Departments.DeptID;

C. SELECT * FROM Employees INNER JOIN Departments USING DeptID;

D. SELECT * FROM Employees INNER JOIN Departments ON Employees.DeptID = Departments.DeptID;

Solution

1. Step 1: Review INNER JOIN syntax

   The correct syntax uses INNER JOIN with ON and a single equals sign (=) for comparison.

2. Step 2: Check each option

   SELECT * FROM Employees INNER JOIN Departments ON Employees.DeptID = Departments.DeptID; uses correct INNER JOIN syntax with ON and =. SELECT * FROM Employees JOIN Departments WHERE Employees.DeptID = Departments.DeptID; uses WHERE instead of ON. SELECT * FROM Employees INNER JOIN Departments USING DeptID; uses USING instead of ON. SELECT * FROM Employees INNER JOIN Departments ON Employees.DeptID == Departments.DeptID; uses double equals (==), which is not valid in SQL.

3. Final Answer:

   SELECT * FROM Employees INNER JOIN Departments ON Employees.DeptID = Departments.DeptID; -> Option D

4. Quick Check:

   INNER JOIN syntax = ON with single = [OK]

Hint: Use ON with single = for INNER JOIN conditions [OK]

Common Mistakes:

• Using WHERE instead of ON for join condition
• Using double equals (==) instead of single equals (=)
• Using USING instead of ON

3. Given the tables:
Employees(id, name, dept_id)
Departments(dept_id, dept_name)
What is the result of this query?

SELECT name, dept_name FROM Employees INNER JOIN Departments ON Employees.dept_id = Departments.dept_id;

Assuming:
Employees: (1, 'Alice', 10), (2, 'Bob', 20), (3, 'Carol', 30)
Departments: (10, 'HR'), (20, 'Sales')

medium

A. [('Alice', 'HR'), ('Bob', 'Sales')]

B. [('Alice', 'HR'), ('Bob', 'Sales'), ('Carol', '30')]

C. [('Alice', 'HR'), ('Bob', 'Sales'), ('Carol', NULL)]

D. [('Alice', 'HR'), ('Bob', 'Sales'), ('Carol', 'Finance')]

Solution

1. Step 1: Identify matching rows by dept_id

   Employees with dept_id 10 and 20 match Departments with same dept_id. Carol's dept_id 30 has no match.

2. Step 2: Understand INNER JOIN output

   INNER JOIN returns only rows with matching dept_id in both tables, so Carol is excluded.

3. Final Answer:

   [('Alice', 'HR'), ('Bob', 'Sales')] -> Option A

4. Quick Check:

   INNER JOIN excludes unmatched rows [OK]

Hint: INNER JOIN excludes rows without matching keys [OK]

Common Mistakes:

• Including unmatched rows in result
• Assuming NULL values appear for unmatched rows
• Confusing INNER JOIN with LEFT JOIN behavior

4. Identify the error in this SQL query:

SELECT e.name, d.dept_name FROM Employees e INNER JOIN Departments d ON e.dept_id == d.dept_id;

medium

A. The join condition uses '==' instead of '='.

B. Using alias names for tables is not allowed.

C. Missing WHERE clause for filtering.

D. INNER JOIN requires USING instead of ON.

Solution

1. Step 1: Check join condition syntax

   SQL uses a single equals sign (=) for comparison, not double equals (==).

2. Step 2: Verify other parts of the query

   Aliases e and d are valid. WHERE clause is optional. INNER JOIN can use ON.

3. Final Answer:

   The join condition uses '==' instead of '='. -> Option A

4. Quick Check:

   Use = for join conditions, not == [OK]

Hint: Use single = in ON clause, not double == [OK]

Common Mistakes:

• Using == instead of = in join condition
• Thinking aliases are disallowed
• Confusing ON with WHERE clause necessity

5. You have two tables:
Orders(order_id, customer_id, amount)
Customers(customer_id, customer_name)
You want to find all customers who have placed orders and the total amount they spent.
Which query correctly uses INNER JOIN and aggregation to get this result?

hard

A. SELECT c.customer_name, SUM(o.amount) AS total_spent FROM Customers c LEFT JOIN Orders o ON c.customer_id = o.customer_id GROUP BY c.customer_name;

B. SELECT c.customer_name, o.amount FROM Customers c INNER JOIN Orders o ON c.customer_id = o.customer_id;

C. SELECT c.customer_name, SUM(o.amount) AS total_spent FROM Customers c INNER JOIN Orders o ON c.customer_id = o.customer_id GROUP BY c.customer_name;

D. SELECT customer_name, amount FROM Customers INNER JOIN Orders ON customer_id = customer_id;

Solution

1. Step 1: Understand the requirement

   We want customers who placed orders and the total amount spent, so INNER JOIN with aggregation is needed.

2. Step 2: Analyze each option

   SELECT c.customer_name, SUM(o.amount) AS total_spent FROM Customers c INNER JOIN Orders o ON c.customer_id = o.customer_id GROUP BY c.customer_name; correctly uses INNER JOIN on customer_id and sums amounts grouped by customer_name. SELECT c.customer_name, SUM(o.amount) AS total_spent FROM Customers c LEFT JOIN Orders o ON c.customer_id = o.customer_id GROUP BY c.customer_name; uses LEFT JOIN, which includes customers without orders. SELECT c.customer_name, o.amount FROM Customers c INNER JOIN Orders o ON c.customer_id = o.customer_id; does not aggregate amounts. SELECT customer_name, amount FROM Customers INNER JOIN Orders ON customer_id = customer_id; has ambiguous join condition and no aggregation.

3. Final Answer:

   SELECT c.customer_name, SUM(o.amount) AS total_spent FROM Customers c INNER JOIN Orders o ON c.customer_id = o.customer_id GROUP BY c.customer_name; -> Option C

4. Quick Check:

   INNER JOIN + GROUP BY + SUM = total spent per customer [OK]

Hint: Use INNER JOIN with GROUP BY and SUM for totals [OK]

Common Mistakes:

• Using LEFT JOIN instead of INNER JOIN when only matching rows needed
• Forgetting GROUP BY with aggregation
• Incorrect join condition syntax