Why advanced joins matter in SQL - Challenge Your Understanding

Challenge - 5 Problems

❓ query_result

intermediate

Output of LEFT JOIN with NULL values

Consider two tables: Employees and Departments. Employees may or may not belong to a department.

What is the output of this query?

SELECT e.name, d.department_name
FROM Employees e
LEFT JOIN Departments d ON e.department_id = d.id
ORDER BY e.name;

SQL

CREATE TABLE Employees (id INT, name VARCHAR(20), department_id INT);
CREATE TABLE Departments (id INT, department_name VARCHAR(20));
INSERT INTO Employees VALUES (1, 'Alice', 10), (2, 'Bob', NULL), (3, 'Charlie', 20);
INSERT INTO Departments VALUES (10, 'HR'), (20, 'IT');

A [('Alice', 'HR'), ('Bob', 'HR'), ('Charlie', 'IT')]

B [('Alice', 'HR'), ('Bob', NULL), ('Charlie', 'IT')]

C [('Alice', 'HR'), ('Charlie', 'IT')]

D [('Bob', NULL)]

💡 Hint

LEFT JOIN keeps all rows from the left table even if there is no matching row in the right table.

The LEFT JOIN returns all employees. For Bob, who has no department_id, the department_name is NULL.

🧠 Conceptual

intermediate

Why use FULL OUTER JOIN?

Which situation best explains why a FULL OUTER JOIN is useful?

A To get all records from the left table and matching from the right only.

B To get only matching records from both tables.

C To get all records from both tables, including unmatched rows from either side.

D To get all records from the right table and matching from the left only.

💡 Hint

Think about when you want to see everything from both tables, even if no match exists.

FULL OUTER JOIN returns all rows from both tables, filling NULLs where no match exists.

📝 Syntax

advanced

Identify the syntax error in this JOIN query

Which option contains a syntax error in the JOIN clause?

SELECT a.id, b.value
FROM TableA a
JOIN TableB b ON a.id = b.a_id;

A SELECT a.id, b.value FROM TableA a JOIN TableB b WHERE a.id = b.a_id;

B SELECT a.id, b.value FROM TableA a INNER JOIN TableB b ON a.id = b.a_id;

C SELECT a.id, b.value FROM TableA a JOIN TableB b ON a.id = b.a_id;

D SELECT a.id, b.value FROM TableA a LEFT JOIN TableB b ON a.id = b.a_id;

💡 Hint

Check where the join condition is placed.

JOIN requires the ON clause for the join condition, not WHERE.

❓ optimization

advanced

Optimizing JOINs for large tables

You have two large tables and want to join them efficiently. Which approach is best?

A Use INNER JOIN with indexed join columns and filter early in WHERE clause.

B Use CROSS JOIN and filter results with WHERE clause after join.

C Use FULL OUTER JOIN without indexes to get all data.

D Use LEFT JOIN without any filters or indexes.

💡 Hint

Indexes help speed up joins by quickly matching rows.

INNER JOIN with indexes and early filtering reduces data processed and speeds query.

🔧 Debug

expert

Why does this JOIN query return duplicate rows?

Given these tables:

Orders(order_id, customer_id)
Customers(customer_id, name)

Query:

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

Why might this query return duplicate rows for the same order?

A Because the query is missing a GROUP BY clause.

B Because Orders table has duplicate order_id values causing multiple matches.

C Because JOIN syntax is incorrect and duplicates rows by default.

D Because Customers table has duplicate customer_id values causing multiple matches.

💡 Hint

Check if the join keys are unique in both tables.

If Customers has duplicate customer_id, each order matches multiple customers, causing duplicates.

Practice

1. Which type of SQL join returns only the rows that have matching values in both tables?

easy

A. INNER JOIN

B. LEFT JOIN

C. RIGHT JOIN

D. FULL JOIN

Solution

1. Step 1: Understand INNER JOIN behavior

   INNER JOIN returns rows where the join condition matches in both tables, excluding unmatched rows.

2. Step 2: Compare with other joins

   LEFT JOIN returns all rows from the left table, RIGHT JOIN from the right, and FULL JOIN all rows from both tables, including unmatched ones.

3. Final Answer:

   INNER JOIN -> Option A

4. Quick Check:

   Matching rows only = INNER JOIN [OK]

Hint: INNER JOIN = only matched rows from both tables [OK]

Common Mistakes:

• Confusing LEFT JOIN with INNER JOIN
• Thinking FULL JOIN returns only matched rows
• Assuming RIGHT JOIN excludes unmatched rows

2. Which of the following is the correct syntax to perform a LEFT JOIN between tables employees and departments on employees.dept_id = departments.id?

easy

A. SELECT * FROM employees JOIN departments ON employees.dept_id = departments.id LEFT;

B. SELECT * FROM employees JOIN departments WHERE employees.dept_id = departments.id LEFT;

C. SELECT * FROM employees LEFT JOIN departments WHERE employees.dept_id = departments.id;

D. SELECT * FROM employees LEFT JOIN departments ON employees.dept_id = departments.id;

Solution

1. Step 1: Identify correct JOIN syntax

   The correct syntax for LEFT JOIN uses the ON keyword to specify join condition: LEFT JOIN table ON condition.

2. Step 2: Check each option

   SELECT * FROM employees LEFT JOIN departments ON employees.dept_id = departments.id; uses correct syntax. Options B and D misuse WHERE or place LEFT incorrectly. SELECT * FROM employees LEFT JOIN departments WHERE employees.dept_id = departments.id; uses WHERE instead of ON.

3. Final Answer:

   SELECT * FROM employees LEFT JOIN departments ON employees.dept_id = departments.id; -> Option D

4. Quick Check:

   LEFT JOIN requires ON, not WHERE [OK]

Hint: Use ON for join condition, not WHERE in JOIN syntax [OK]

Common Mistakes:

• Using WHERE instead of ON for join condition
• Placing LEFT keyword after JOIN incorrectly
• Omitting ON clause in JOIN

3. Given tables orders and customers, what will the following query return?

SELECT customers.name, orders.id FROM customers LEFT JOIN orders ON customers.id = orders.customer_id WHERE orders.id IS NULL;

medium

A. All customers who have not placed any orders

B. All customers who have placed at least one order

C. All orders without a matching customer

D. All customers and their orders

Solution

1. Step 1: Analyze LEFT JOIN with WHERE condition

   The LEFT JOIN returns all customers with matching orders or NULL if no order exists. The WHERE clause filters rows where orders.id is NULL, meaning no matching order.

2. Step 2: Interpret the result

   This query returns customers who have no orders because orders.id is NULL for them.

3. Final Answer:

   All customers who have not placed any orders -> Option A

4. Quick Check:

   LEFT JOIN + WHERE orders.id IS NULL = customers without orders [OK]

Hint: LEFT JOIN + WHERE right table column IS NULL finds missing matches [OK]

Common Mistakes:

• Thinking it returns customers with orders
• Confusing NULL in orders.id with existing orders
• Assuming it returns unmatched orders

4. Consider this SQL query:

SELECT a.id, b.value FROM tableA a RIGHT JOIN tableB b ON a.id = b.a_id WHERE a.id > 10;

What is the main issue with this query?

medium

A. RIGHT JOIN syntax is incorrect; it should be LEFT JOIN

B. The WHERE clause filters out rows where a.id is NULL, negating the RIGHT JOIN effect

C. The ON condition is invalid because columns have different names

D. The query will cause a syntax error due to aliasing

Solution

1. Step 1: Understand RIGHT JOIN with WHERE filter

   RIGHT JOIN returns all rows from tableB and matching from tableA. Rows with no match have NULL in a.id.

2. Step 2: Effect of WHERE a.id > 10

   The WHERE clause excludes rows where a.id is NULL, removing unmatched rows from tableB, which defeats the purpose of RIGHT JOIN.

3. Final Answer:

   The WHERE clause filters out rows where a.id is NULL, negating the RIGHT JOIN effect -> Option B

4. Quick Check:

   Filtering NULLs after RIGHT JOIN removes unmatched rows [OK]

Hint: Filter NULLs in JOIN condition, not WHERE, to keep unmatched rows [OK]

Common Mistakes:

• Thinking RIGHT JOIN syntax is wrong
• Ignoring NULL filtering effect in WHERE
• Assuming aliasing causes error

5. You have two tables: students (id, name) and enrollments (student_id, course). You want to list all students and the courses they are enrolled in, including students with no enrollments. Which SQL query correctly achieves this?

hard

A. SELECT students.name, enrollments.course FROM students INNER JOIN enrollments ON students.id = enrollments.student_id;

B. SELECT students.name, enrollments.course FROM enrollments LEFT JOIN students ON students.id = enrollments.student_id;

C. SELECT students.name, enrollments.course FROM students LEFT JOIN enrollments ON students.id = enrollments.student_id;

D. SELECT students.name, enrollments.course FROM students RIGHT JOIN enrollments ON students.id = enrollments.student_id;

Solution

1. Step 1: Identify requirement for all students

   We want all students listed, even those without enrollments, so the join must keep all rows from students.

2. Step 2: Choose correct join type

   LEFT JOIN keeps all rows from the left table (students) and matches enrollments if any. INNER JOIN excludes students without enrollments. RIGHT JOIN would keep all enrollments, not students.

3. Final Answer:

   SELECT students.name, enrollments.course FROM students LEFT JOIN enrollments ON students.id = enrollments.student_id; -> Option C

4. Quick Check:

   LEFT JOIN keeps all left table rows (students) [OK]

Hint: Use LEFT JOIN to keep all from first table, even if no match [OK]

Common Mistakes:

• Using INNER JOIN excludes students without courses
• Using RIGHT JOIN keeps all enrollments, not students
• Swapping table order changes join meaning