Joining more than two tables in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to join two tables on the customer_id column.

SQL

SELECT orders.order_id, customers.name FROM orders JOIN [1] ON orders.customer_id = customers.customer_id;

Drag options to blanks, or click blank then click option'

Asuppliers

Bproducts

Cemployees

Dcustomers

Attempts:

3 left

💡 Hint

Common Mistakes

Joining with the wrong table like products or employees.

Forgetting to specify the join condition.

The customers table is joined to orders using the customer_id column to get customer names for each order.

2fill in blank

medium

Complete the code to join three tables: orders, customers, and products.

SQL

SELECT orders.order_id, customers.name, products.product_name FROM orders JOIN customers ON orders.customer_id = customers.customer_id JOIN [1] ON orders.product_id = products.product_id;

Drag options to blanks, or click blank then click option'

Acategories

Bproducts

Csuppliers

Demployees

Attempts:

3 left

💡 Hint

Common Mistakes

Joining with employees or suppliers instead of products.

Using wrong join conditions.

The products table is joined to orders using the product_id column to get product names for each order.

3fill in blank

hard

Fix the error in the join condition to correctly join orders and customers.

SQL

SELECT orders.order_id, customers.name FROM orders JOIN customers ON orders.[1] = customers.customer_id;

Drag options to blanks, or click blank then click option'

Acustomer_id

Border_id

Cproduct_id

Demployee_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using order_id instead of customer_id in the join condition.

Joining on unrelated columns like product_id or employee_id.

The join condition must match the customer_id in both tables to link orders to customers correctly.

4fill in blank

hard

Fill both blanks to join orders, customers, and products correctly.

SQL

SELECT orders.order_id, customers.name, products.product_name FROM orders JOIN [1] ON orders.customer_id = customers.customer_id JOIN [2] ON orders.product_id = products.product_id;

Drag options to blanks, or click blank then click option'

Acustomers

Bemployees

Cproducts

Dsuppliers

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the order of tables in joins.

Joining with unrelated tables like employees or suppliers.

First, join customers to get customer names, then join products to get product names for each order.

5fill in blank

hard

Fill all three blanks to join orders, customers, and products and filter orders with quantity greater than 5.

SQL

SELECT orders.order_id, customers.name, products.product_name FROM orders JOIN [1] ON orders.customer_id = customers.customer_id JOIN [2] ON orders.product_id = products.product_id WHERE orders.[3] > 5;

Drag options to blanks, or click blank then click option'

Acustomers

Bquantity

Cproducts

Demployee_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong table names in joins.

Filtering on a column that does not exist or is unrelated.

Join customers and products tables to orders, then filter orders where the quantity is greater than 5.

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