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.

Practice

1. Why do we need LEFT OUTER JOIN in SQL?

easy

A. To include all rows from the left table even if there is no matching row in the right table

B. To only show rows that have matching values in both tables

C. To delete rows from the left table

D. To update rows in the right table

Solution

1. Step 1: Understand the purpose of LEFT OUTER JOIN

   LEFT OUTER JOIN returns all rows from the left table and matched rows from the right table. If no match, NULLs appear for right table columns.

2. Step 2: Compare with INNER JOIN behavior

   INNER JOIN returns only rows with matching keys in both tables, excluding unmatched rows.

3. Final Answer:

   To include all rows from the left table even if there is no matching row in the right table -> Option A

4. Quick Check:

   LEFT OUTER JOIN shows all left rows [OK]

Hint: LEFT OUTER JOIN keeps all left rows, unmatched get NULLs [OK]

Common Mistakes:

• Confusing LEFT OUTER JOIN with INNER JOIN
• Thinking it deletes or updates rows
• Assuming it only shows matched rows

2. Which of the following is the correct syntax for a LEFT OUTER JOIN?

easy

A. SELECT * FROM table1 JOIN LEFT OUTER table2 ON table1.id = table2.id;

B. SELECT * FROM table1 LEFT JOIN table2 ON table1.id = table2.id;

C. SELECT * FROM table1 OUTER LEFT JOIN table2 ON table1.id = table2.id;

D. SELECT * FROM table1 LEFT OUTER JOIN table2 WHERE table1.id = table2.id;

Solution

1. Step 1: Recall correct LEFT OUTER JOIN syntax

   The correct syntax is: SELECT ... FROM table1 LEFT JOIN table2 ON condition; LEFT JOIN is shorthand for LEFT OUTER JOIN.

2. Step 2: Identify syntax errors in other options

   SELECT * FROM table1 JOIN LEFT OUTER table2 ON table1.id = table2.id; has JOIN LEFT OUTER which is invalid order. SELECT * FROM table1 OUTER LEFT JOIN table2 ON table1.id = table2.id; uses OUTER LEFT JOIN which is incorrect. SELECT * FROM table1 LEFT OUTER JOIN table2 WHERE table1.id = table2.id; uses WHERE instead of ON for join condition.

3. Final Answer:

   SELECT * FROM table1 LEFT JOIN table2 ON table1.id = table2.id; -> Option B

4. Quick Check:

   LEFT JOIN ... ON ... is correct syntax [OK]

Hint: Use LEFT JOIN ... ON ... for correct outer join syntax [OK]

Common Mistakes:

• Swapping JOIN and LEFT keywords
• Using WHERE instead of ON for join condition
• Writing OUTER LEFT JOIN instead of LEFT OUTER JOIN

3. Given tables Employees and Departments where some employees have no department, what will this query return?

SELECT e.name, d.name FROM Employees e LEFT JOIN Departments d ON e.dept_id = d.id;

medium

A. All departments with employee names; NULL for departments without employees

B. Only employees who have a matching department

C. All employees with their department names; NULL for employees without a department

D. Only departments with employees

Solution

1. Step 1: Analyze LEFT JOIN behavior on Employees and Departments

   LEFT JOIN keeps all rows from Employees (left table). For employees without matching department, department columns show NULL.

2. Step 2: Understand output columns

   Query selects employee name and department name. Employees without department show NULL in department name.

3. Final Answer:

   All employees with their department names; NULL for employees without a department -> Option C

4. Quick Check:

   LEFT JOIN keeps all left rows, unmatched right columns NULL [OK]

Hint: LEFT JOIN shows all left rows, unmatched right side NULL [OK]

Common Mistakes:

• Thinking only matched rows appear
• Confusing left and right tables
• Expecting departments without employees to appear

4. What is wrong with this query if we want to list all customers and their orders, including customers with no orders?

SELECT c.name, o.order_id FROM Customers c INNER JOIN Orders o ON c.id = o.customer_id;

medium

A. INNER JOIN excludes customers without orders; should use LEFT OUTER JOIN

B. The ON clause is missing

C. The SELECT statement is missing table aliases

D. Orders table should be first in the FROM clause

Solution

1. Step 1: Understand INNER JOIN behavior

   INNER JOIN returns only rows with matching keys in both tables, so customers without orders are excluded.

2. Step 2: Identify correct join for including all customers

   LEFT OUTER JOIN keeps all customers even if no matching orders exist, showing NULL for order columns.

3. Final Answer:

   INNER JOIN excludes customers without orders; should use LEFT OUTER JOIN -> Option A

4. Quick Check:

   Use LEFT OUTER JOIN to include all left table rows [OK]

Hint: Use LEFT OUTER JOIN to include unmatched left table rows [OK]

Common Mistakes:

• Using INNER JOIN when outer join is needed
• Forgetting ON clause (though present here)
• Thinking table order in FROM matters for join type

5. You have two tables: Students and Enrollments. Some students are not enrolled in any course. Which query correctly lists all students and their courses, showing NULL for students without enrollments?

hard

A. SELECT s.name, e.course FROM Students s INNER JOIN Enrollments e ON s.id = e.student_id;

B. SELECT s.name, e.course FROM Students s RIGHT OUTER JOIN Enrollments e ON s.id = e.student_id;

C. SELECT s.name, e.course FROM Enrollments e LEFT OUTER JOIN Students s ON s.id = e.student_id;

D. SELECT s.name, e.course FROM Students s LEFT OUTER JOIN Enrollments e ON s.id = e.student_id;

Solution

1. Step 1: Identify which table has all students

   Students table contains all students, including those without enrollments.

2. Step 2: Choose join to keep all students

   LEFT OUTER JOIN with Students as left table keeps all students, adding course info or NULL if no enrollment.

3. Step 3: Evaluate other options

   INNER JOIN excludes students without enrollments. RIGHT OUTER JOIN with Enrollments left excludes students without enrollments. LEFT OUTER JOIN with Enrollments left table excludes students without enrollments.

4. Final Answer:

   SELECT s.name, e.course FROM Students s LEFT OUTER JOIN Enrollments e ON s.id = e.student_id; -> Option D

5. Quick Check:

   LEFT OUTER JOIN keeps all left table rows [OK]

Hint: LEFT OUTER JOIN with Students on left keeps all students [OK]

Common Mistakes:

• Using INNER JOIN excludes students without courses
• Swapping left and right tables in join
• Using RIGHT OUTER JOIN incorrectly