Why outer joins are needed in SQL - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all customers and their orders, including customers with no orders.

SQL

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

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

ARIGHT JOIN

BLEFT OUTER JOIN

CINNER JOIN

DCROSS JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using INNER JOIN excludes customers without orders.

Using CROSS JOIN creates all combinations, not what we want.

LEFT OUTER JOIN returns all rows from the left table (customers) and matching rows from the right table (orders). It includes customers without orders.

2fill in blank

medium

Complete the code to select all orders and their customers, including orders without customers.

SQL

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

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

AINNER JOIN

BFULL JOIN

CRIGHT OUTER JOIN

DLEFT JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using INNER JOIN excludes orders without customers.

Using RIGHT OUTER JOIN keeps all rows from the right table, not orders.

LEFT JOIN returns all rows from the left table (orders) and matching rows from the right table (customers). It includes orders without customers.

3fill in blank

hard

Fix the error in the query to include all customers and orders, even if no match exists.

SQL

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

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

ARIGHT

BINNER

CLEFT

DFULL OUTER

Attempts:

3 left

💡 Hint

Common Mistakes

Using INNER JOIN excludes customers without orders.

Using RIGHT JOIN keeps all orders, not customers.

LEFT JOIN includes all customers and matching orders, showing NULL for customers without orders.

4fill in blank

hard

Fill both blanks to select all customers and orders, showing NULL where no match exists.

SQL

SELECT [1].name, [2].order_id FROM customers [1] LEFT JOIN orders [2] ON [1].id = [2].customer_id;

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

Ac

Bo

Ccustomers

Dorders

Attempts:

3 left

💡 Hint

Common Mistakes

Using table names without alias when aliases are used in JOIN.

Mixing aliases and full table names incorrectly.

Using aliases 'c' for customers and 'o' for orders makes the query clear and correct.

5fill in blank

hard

Fill all three blanks to select all customers and orders, including unmatched rows, using FULL OUTER JOIN.

SQL

SELECT [1].name, [2].order_id FROM [1] FULL OUTER JOIN [3] ON [1].id = [3].customer_id;

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

Acustomers

Borders

Attempts:

3 left

💡 Hint

Common Mistakes

Using INNER JOIN excludes unmatched rows.

Mixing table names incorrectly in ON clause.

FULL OUTER JOIN returns all customers and orders, matching where possible and showing NULL where no match exists.