Joining more than two tables in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of joining three tables with INNER JOIN

Given the tables Employees, Departments, and Locations, what is the output of the following SQL query?

SELECT e.name, d.department_name, l.city
FROM Employees e
INNER JOIN Departments d ON e.department_id = d.id
INNER JOIN Locations l ON d.location_id = l.id
ORDER BY e.name;

SQL

CREATE TABLE Employees (id INT, name VARCHAR(50), department_id INT);
CREATE TABLE Departments (id INT, department_name VARCHAR(50), location_id INT);
CREATE TABLE Locations (id INT, city VARCHAR(50));

INSERT INTO Employees VALUES (1, 'Alice', 10), (2, 'Bob', 20), (3, 'Charlie', 10);
INSERT INTO Departments VALUES (10, 'Sales', 100), (20, 'Engineering', 200);
INSERT INTO Locations VALUES (100, 'New York'), (200, 'San Francisco');

A [{'name': 'Alice', 'department_name': 'Sales', 'city': 'New York'}, {'name': 'Bob', 'department_name': 'Engineering', 'city': 'San Francisco'}, {'name': 'Charlie', 'department_name': 'Sales', 'city': 'New York'}]

B [{'name': 'Alice', 'department_name': 'Sales', 'city': 'San Francisco'}, {'name': 'Bob', 'department_name': 'Engineering', 'city': 'New York'}, {'name': 'Charlie', 'department_name': 'Sales', 'city': 'San Francisco'}]

C [{'name': 'Alice', 'department_name': 'Sales', 'city': 'New York'}, {'name': 'Charlie', 'department_name': 'Sales', 'city': 'New York'}]

D [{'name': 'Bob', 'department_name': 'Engineering', 'city': 'San Francisco'}]

💡 Hint

Think about how INNER JOIN works: it only includes rows where the join condition matches in all tables.

The query joins Employees to Departments on department_id, then Departments to Locations on location_id. All employees have matching departments and locations, so all three employees appear with their correct department and city.

📝 Syntax

intermediate

Identify the syntax error in joining three tables

Which option contains a syntax error when joining three tables Orders, Customers, and Products to get order details?

SQL

SELECT o.order_id, c.customer_name, p.product_name
FROM Orders o
JOIN Customers c ON o.customer_id = c.id
JOIN Products p ON o.product_id = p.id;

A SELECT o.order_id, c.customer_name, p.product_name FROM Orders o INNER JOIN Customers c ON o.customer_id = c.id JOIN Products p ON o.product_id = p.id;

B SELECT o.order_id, c.customer_name, p.product_name FROM Orders o JOIN Customers c ON o.customer_id = c.id JOIN Products p o.product_id = p.id;

C SELECT o.order_id, c.customer_name, p.product_name FROM Orders o JOIN Customers c ON o.customer_id = c.id INNER JOIN Products p ON o.product_id = p.id;

D SELECT o.order_id, c.customer_name, p.product_name FROM Orders o JOIN Customers c ON o.customer_id = c.id JOIN Products p ON o.product_id = p.id;

💡 Hint

Check the JOIN syntax carefully, especially the ON clause.

Option B is missing the ON keyword before the join condition for Products, causing a syntax error.

❓ optimization

advanced

Optimizing a query joining four tables

You have four tables: Sales, Customers, Products, and Stores. Which query is the most efficient to get sales details including customer name, product name, and store location?

A SELECT s.sale_id, c.name, p.name, st.location FROM Sales s LEFT JOIN Customers c ON s.customer_id = c.id LEFT JOIN Products p ON s.product_id = p.id LEFT JOIN Stores st ON s.store_id = st.id;

B SELECT s.sale_id, c.name, p.name, st.location FROM Sales s, Customers c, Products p, Stores st WHERE s.customer_id = c.id AND s.product_id = p.id AND s.store_id = st.id;

C SELECT s.sale_id, c.name, p.name, st.location FROM Sales s JOIN Customers c ON s.customer_id = c.id JOIN Products p ON s.product_id = p.id JOIN Stores st ON s.store_id = st.id;

D SELECT s.sale_id, c.name, p.name, st.location FROM Sales s CROSS JOIN Customers c CROSS JOIN Products p CROSS JOIN Stores st WHERE s.customer_id = c.id AND s.product_id = p.id AND s.store_id = st.id;

💡 Hint

Consider the clarity and performance of JOIN syntax versus older comma joins and CROSS JOINs.

Option C uses explicit INNER JOINs which are clearer and often optimized better by SQL engines. Option C uses old-style joins which can be less efficient. Option C uses LEFT JOINs which may return extra rows if no matches exist. Option C uses CROSS JOINs which produce large intermediate results.

🧠 Conceptual

advanced

Understanding join types with three tables

You join three tables: A, B, and C. If you want to keep all rows from A even if there are no matching rows in B or C, which join type should you use?

A Use INNER JOIN between A and B, then INNER JOIN between B and C.

B Use FULL OUTER JOIN between A and B, then FULL OUTER JOIN between B and C.

C Use RIGHT JOIN from A to B, then RIGHT JOIN from B to C.

D Use LEFT JOIN from A to B, then LEFT JOIN from B to C.

💡 Hint

Think about which join keeps all rows from the left table regardless of matches.

LEFT JOIN keeps all rows from the left table (A), even if there are no matches in the right tables (B and C). INNER JOIN would exclude unmatched rows.

🔧 Debug

expert

Debugging incorrect results from joining three tables

You run this query joining three tables but get fewer rows than expected:

SELECT e.name, d.department_name, p.project_name
FROM Employees e
JOIN Departments d ON e.department_id = d.id
JOIN Projects p ON e.id = p.employee_id
WHERE d.location = 'New York';

What is the most likely reason for missing rows?

A Some employees in New York departments have no projects, so INNER JOIN excludes them.

B The WHERE clause filters out employees not in New York, causing no rows to show.

C The JOIN condition between Employees and Departments is incorrect, causing no matches.

D The Projects table has no employee_id column, causing a syntax error.

💡 Hint

Consider how INNER JOIN behaves when there is no matching row in the joined table.

INNER JOIN only returns rows where matching rows exist in all joined tables. Employees without projects are excluded, causing fewer rows.

Practice

1. What is the main purpose of joining more than two tables in SQL?

easy

A. To combine related data from multiple tables into one result set

B. To delete data from multiple tables at once

C. To create new tables automatically

D. To backup tables in the database

Solution

1. Step 1: Understand the concept of JOIN

   JOIN is used to combine rows from two or more tables based on related columns.

2. Step 2: Apply to multiple tables

   Joining more than two tables extends this idea to combine data from several tables into one result.

3. Final Answer:

   To combine related data from multiple tables into one result set -> Option A

4. Quick Check:

   Joining multiple tables = combine data [OK]

Hint: Joining means combining data from tables step-by-step [OK]

Common Mistakes:

• Thinking JOIN deletes or creates tables
• Confusing JOIN with backup or delete operations
• Assuming JOIN works without conditions

2. Which of the following is the correct syntax to join three tables A, B, and C on columns A.id = B.a_id and B.id = C.b_id?

easy

A. SELECT * FROM A, B, C WHERE A.id = B.a_id, B.id = C.b_id;

B. SELECT * FROM A JOIN B ON A.id = B.a_id, C ON B.id = C.b_id;

C. SELECT * FROM A JOIN B JOIN C ON A.id = B.a_id AND B.id = C.b_id;

D. SELECT * FROM A JOIN B ON A.id = B.a_id JOIN C ON B.id = C.b_id;

Solution

1. Step 1: Check JOIN syntax for multiple tables

   Each JOIN must have its own ON condition to specify how tables connect.

2. Step 2: Validate SELECT * FROM A JOIN B ON A.id = B.a_id JOIN C ON B.id = C.b_id;

   SELECT * FROM A JOIN B ON A.id = B.a_id JOIN C ON B.id = C.b_id; correctly joins A to B with ON, then B to C with ON separately.

3. Final Answer:

   SELECT * FROM A JOIN B ON A.id = B.a_id JOIN C ON B.id = C.b_id; -> Option D

4. Quick Check:

   Each JOIN needs its own ON condition [OK]

Hint: Use separate ON for each JOIN clause [OK]

Common Mistakes:

• Combining multiple ON conditions in one JOIN
• Using commas with JOIN incorrectly
• Missing ON clause for a JOIN

3. Given tables:
Students(id, name),
Enrollments(student_id, course_id),
Courses(id, title)
What will the query below return?

SELECT Students.name, Courses.title FROM Students JOIN Enrollments ON Students.id = Enrollments.student_id JOIN Courses ON Enrollments.course_id = Courses.id;

medium

A. List of all students and all courses regardless of enrollment

B. List of student names with the titles of courses they are enrolled in

C. List of courses with no students enrolled

D. Syntax error due to missing WHERE clause

Solution

1. Step 1: Analyze JOINs in the query

   Students join Enrollments on student ID, then Enrollments join Courses on course ID, linking students to their courses.

2. Step 2: Understand SELECT output

   The query selects student names and course titles for matching enrollments, showing which student is in which course.

3. Final Answer:

   List of student names with the titles of courses they are enrolled in -> Option B

4. Quick Check:

   JOINs link students to their courses [OK]

Hint: JOIN chains link related data stepwise [OK]

Common Mistakes:

• Thinking JOIN returns all combinations without conditions
• Expecting courses without students to appear
• Assuming WHERE is required for JOIN

4. Identify the error in the following SQL query joining three tables:

SELECT * FROM A JOIN B ON A.id = B.a_id JOIN C ON A.id = C.a_id;

medium

A. Missing WHERE clause for filtering

B. No error, query is correct

C. JOIN condition for table C should use B's columns, not A's

D. JOIN keyword is missing before table C

Solution

1. Step 1: Review JOIN conditions

   First JOIN connects A and B on A.id = B.a_id, which is correct.

2. Step 2: Check second JOIN condition

   Second JOIN connects C using A.id = C.a_id, but logically C should join via B, not A, to maintain correct relationships.

3. Final Answer:

   JOIN condition for table C should use B's columns, not A's -> Option C

4. Quick Check:

   JOIN conditions must link correct tables [OK]

Hint: Check JOIN ON uses correct table columns [OK]

Common Mistakes:

• Using wrong table columns in JOIN condition
• Assuming WHERE is needed for JOIN
• Ignoring logical table relationships

5. You have tables:
Orders(order_id, customer_id),
Customers(customer_id, name),
Payments(payment_id, order_id, amount).
Write a query to find each customer's name and the total amount they paid across all orders. Which query is correct?

hard

A. SELECT Customers.name, SUM(Payments.amount) FROM Customers JOIN Orders ON Customers.customer_id = Orders.customer_id JOIN Payments ON Orders.order_id = Payments.order_id GROUP BY Customers.name;

B. SELECT Customers.name, Payments.amount FROM Customers JOIN Orders ON Customers.customer_id = Orders.customer_id JOIN Payments ON Orders.order_id = Payments.order_id;

C. SELECT Customers.name, SUM(Payments.amount) FROM Customers, Orders, Payments WHERE Customers.customer_id = Orders.customer_id AND Orders.order_id = Payments.order_id;

D. SELECT Customers.name, SUM(Payments.amount) FROM Customers JOIN Payments ON Customers.customer_id = Payments.order_id GROUP BY Customers.name;

Solution

1. Step 1: Understand the relationships

   Customers link to Orders by customer_id; Orders link to Payments by order_id.

2. Step 2: Check aggregation and grouping

   We need total payment per customer, so SUM and GROUP BY Customers.name are required.

3. Step 3: Validate options

   SELECT Customers.name, SUM(Payments.amount) FROM Customers JOIN Orders ON Customers.customer_id = Orders.customer_id JOIN Payments ON Orders.order_id = Payments.order_id GROUP BY Customers.name; correctly joins tables and groups by customer name with SUM of payments. SELECT Customers.name, Payments.amount FROM Customers JOIN Orders ON Customers.customer_id = Orders.customer_id JOIN Payments ON Orders.order_id = Payments.order_id; lacks aggregation. SELECT Customers.name, SUM(Payments.amount) FROM Customers, Orders, Payments WHERE Customers.customer_id = Orders.customer_id AND Orders.order_id = Payments.order_id; misses GROUP BY. SELECT Customers.name, SUM(Payments.amount) FROM Customers JOIN Payments ON Customers.customer_id = Payments.order_id GROUP BY Customers.name; joins wrong columns.

4. Final Answer:

   SELECT Customers.name, SUM(Payments.amount) FROM Customers JOIN Orders ON Customers.customer_id = Orders.customer_id JOIN Payments ON Orders.order_id = Payments.order_id GROUP BY Customers.name; -> Option A

5. Quick Check:

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

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

Common Mistakes:

• Missing GROUP BY when using SUM
• Joining tables on wrong columns
• Selecting aggregated and non-aggregated columns without GROUP BY