Test Your SQL LEFT JOIN Knowledge with Our Interactive Quiz

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True / False

In MySQL, a LEFT JOIN returns all rows from the left table and the matching rows from the right table.

True False

A LEFT JOIN returns all rows from the left table, and the matching rows from the right table. If there is no match, the result is NULL on the side of the right table.

True / False

In MySQL, using LEFT JOIN will result in the same output as using INNER JOIN if there are no NULL values in the join column of the left table.

True False

If there are no NULL values in the join column of the left table, LEFT JOIN and INNER JOIN will produce the same results.

True / False

In MySQL, you can only use LEFT JOIN with two tables.

True False

LEFT JOIN can be used with more than two tables in MySQL by chaining multiple JOINs together.

True / False

In MySQL, the LEFT JOIN operation is generally faster than the INNER JOIN operation.

True False

Performance depends on the database structure and indexing. Generally, INNER JOIN can be faster because it only returns matching rows, whereas LEFT JOIN returns all rows from the left table and matches from the right table.

True / False

MySQL LEFT JOIN can be used with the WHERE clause to filter the results.

True False

You can use the WHERE clause to filter the results of a LEFT JOIN in MySQL.

True / False

In MySQL, LEFT JOIN will return duplicate rows if the right table contains multiple matching rows.

True False

LEFT JOIN will return duplicate rows if there are multiple matches in the right table for a row in the left table.

True / False

In MySQL, when using LEFT JOIN, the order of the tables in the JOIN clause does not matter.

True False

The order of tables matters in LEFT JOIN. The first table is the "left" table, and the second is the "right" table. Switching them changes the output.

True / False

In MySQL, LEFT JOIN can be used with subqueries.

True False

You can use LEFT JOIN with subqueries in MySQL to join the results of a subquery with another table.

True / False

In MySQL, LEFT JOIN can be combined with other types of JOINs like RIGHT JOIN and CROSS JOIN in a single query.

True False

Different types of JOINs, including LEFT JOIN, RIGHT JOIN, and CROSS JOIN, can be combined in a single query in MySQL.

Q10

True / False

In MySQL, using LEFT JOIN with a condition in the ON clause will always return all rows from the left table regardless of the condition.

True False

The condition in the ON clause affects the matching of rows. While LEFT JOIN returns all rows from the left table, the ON condition determines how rows from the right table are matched. If no match is found, NULLs are returned for columns from the right table.

Q11

Single Choice

What does a LEFT JOIN in MySQL do?

Returns only rows from the left table that have matching rows in the right table Returns only rows from the right table that have matching rows in the left table Returns all rows from the left table, and the matched rows from the right tableReturns all rows from both tables

A LEFT JOIN returns all rows from the left table, and the matched rows from the right table. If there is no match, the result is NULL on the side of the right table.

Q12

Single Choice

Given two tables, employees and departments, which SQL query will return all employees and their corresponding department names, even if the department is not assigned?

SELECT * FROM employees RIGHT JOIN departments ON employees.dept_id = departments.id; SELECT * FROM employees LEFT JOIN departments ON employees.dept_id = departments.id; SELECT * FROM employees INNER JOIN departments ON employees.dept_id = departments.id;SELECT * FROM employees FULL JOIN departments ON employees.dept_id = departments.id;

LEFT JOIN between employees and departments ensures all employees are returned even if they don't belong to any department. The RIGHT JOIN would do the opposite.

Q13

Single Choice

Which of the following statements is true regarding a LEFT JOIN?

It returns only the rows that have a match in both tables. It returns all rows from the right table and the matched rows from the left table. It returns all rows from the left table and unmatched rows are filled with NULLs in the result set.It is identical to a RIGHT JOIN.

The LEFT JOIN returns all rows from the left table, filling in NULLs where there is no match in the right table.

Q14

Single Choice

Given the following SQL code, what will the result be if departments has a department with id = 4, but there is no employee with dept_id = 4?

SQL Code

SELECT employees.name, departments.name
FROM employees
LEFT JOIN departments ON employees.dept_id = departments.id;

The result will include all employees, and NULL for department where there's no match. The result will exclude the department with id = 4. The result will include all departments, and NULL for employees where there's no match.The query will return an error

The LEFT JOIN includes all employees, even if they don't belong to any department. The department with id = 4 will appear with NULL for the employee's name

Q15

Single Choice

Which of the following SQL queries will return a list of employees with their corresponding department names, and show 'No Department' for employees without a department?

SQL Code

SELECT employees.name, COALESCE(departments.name, 'No Department') AS department_name
FROM employees
_______ JOIN departments ON employees.dept_id = departments.id;

INNER LEFT RIGHTFULL

The LEFT JOIN is required to return all employees, and COALESCE is used to replace NULL values with 'No Department' where there is no match.

Q16

Single Choice

What will the following query return if there are no employees in the 'HR' department?

SQL Code

SELECT employees.name, departments.name
FROM employees
LEFT JOIN departments ON employees.dept_id = departments.id
WHERE departments.name = 'HR';

All employees from all departments All employees from the HR department Only employees from other departmentsAn empty result set

The WHERE clause filters the result after the join, so if there are no employees in 'HR', the result will be empty.

Q17

Single Choice

What is the effect of the ON clause in a LEFT JOIN when the condition references columns that contain NULL values?

It automatically excludes NULL values from the result set. It includes all rows where the condition is TRUE, and fills NULL where there is no match. It includes only the rows where both tables have non-NULL values in the specified columns.It behaves like an INNER JOIN when NULL values are present

In a LEFT JOIN, the ON clause determines which rows are matched. If there is no match, the result set will include NULLs for the columns from the right table.

Q18

Single Choice

Consider the following tables and query. What will the query output?

SQL Code

SELECT customers.name, orders.order_date
FROM customers
LEFT JOIN orders ON customers.id = orders.customer_id
WHERE orders.order_date IS NOT NULL;
customers contains 10 rows with ids from 1 to 10.
orders contains 5 rows with customer_ids 1 to 5.

0 row 5 rows 15 rows20 rows

The LEFT JOIN initially returns all rows from customers, but the WHERE clause filters out the rows where order_date is NULL, leaving only those with orders.

Q19

Single Choice

In a scenario where you are performing a LEFT JOIN on two large tables, how can you optimize performance?

Use indexes on the columns involved in the ON clause. Avoid using LEFT JOIN and use INNER JOIN instead. Remove the WHERE clause to increase speed.Perform the join without any conditions.

Indexes on the columns used in the ON clause can significantly improve the performance of a LEFT JOIN, especially with large tables.

Q20

Single Choice

Given the following SQL code, what will be the result if the projects table has no rows with department_id = 2, and the departments table has a row with id = 2?

SQL Code

SELECT departments.name, COUNT(projects.id) AS project_count
FROM departments
LEFT JOIN projects ON departments.id = projects.department_id
GROUP BY departments.name;

The department with id = 2 will not appear in the result set. The department with id = 2 will appear with project_count = 0. The result set will only include departments with at least one project.The query will return an error because of the GROUP BY clause

The LEFT JOIN ensures that all departments appear in the result, even if they have no projects. The COUNT function will return 0 for departments with no matching rows in the projects table.

Q21

Multiple Choice

Which SQL query uses LEFT JOIN to combine records from the 'employees' and 'departments' tables, returning all employees even if they don't belong to a department?

SQL Code

SELECT e.employee_id, e.name, d.department_name
FROM employees e
LEFT JOIN departments d ON e.department_id = d.department_id;

SELECT e.employee_id, e.name, d.department_name FROM employees e LEFT JOIN departments d ON e.department_id = d.department_id; SELECT e.employee_id, e.name, d.department_name FROM employees e JOIN departments d ON e.department_id = d.department_id; SELECT e.employee_id, e.name, department_name FROM employees e LEFT JOIN departments d ON e.department_id = d.department_id;SELECT e.employee_id, e.name, d.department_name FROM employees e RIGHT JOIN departments d ON e.department_id = d.department_id;

This query combines records from the 'employees' and 'departments' tables using a LEFT JOIN on the 'department_id' column, returning all employees even if they don't belong to any department.

Q22

Multiple Choice

Identify the correct SQL query that uses LEFT JOIN to list all customers and their orders, even if some customers haven't placed any orders.

SQL Code

SELECT c.customer_id, c.customer_name, o.order_id
FROM customers c
LEFT JOIN orders o ON c.customer_id = o.customer_id;

SELECT c.customer_id, c.customer_name, o.order_id FROM customers c LEFT JOIN orders o ON c.customer_id = o.customer_id; SELECT c.customer_id, c.customer_name, o.order_id FROM customers c JOIN orders o ON c.customer_id = o.customer_id; SELECT customer_id, customer_name, order_id FROM customers LEFT JOIN orders ON customers.customer_id = orders.customer_id;SELECT c.customer_id, c.customer_name, o.order_id FROM customers c RIGHT JOIN orders o ON c.customer_id = o.customer_id;

The LEFT JOIN is used to combine rows from the 'customers' and 'orders' tables, returning all customers, including those who haven't placed any orders.

Q23

Multiple Choice

Which SQL query uses LEFT JOIN to retrieve all products and their respective categories, including products that don't belong to any category?

SQL Code

SELECT p.product_id, p.product_name, c.category_name
FROM products p
LEFT JOIN categories c ON p.category_id = c.category_id;

SELECT p.product_id, p.product_name, c.category_name FROM products p LEFT JOIN categories c ON p.category_id = c.category_id; SELECT p.product_id, p.product_name, c.category_name FROM products p JOIN categories c ON p.category_id = c.category_id; SELECT product_id, product_name, category_name FROM products LEFT JOIN categories ON products.category_id = categories.category_id;SELECT p.product_id, p.product_name, c.category_name FROM products p RIGHT JOIN categories c ON p.category_id = c.category_id;

The LEFT JOIN is used to retrieve all products along with their category names, including those that don't belong to any category.

Q24

Multiple Choice

Determine the correct SQL query that uses LEFT JOIN to fetch all students and their respective course names, even if some students are not enrolled in any courses.

SQL Code

SELECT s.student_id, s.student_name, c.course_name
FROM students s
LEFT JOIN courses c ON s.course_id = c.course_id;

SELECT s.student_id, s.student_name, c.course_name FROM students s LEFT JOIN courses c ON s.course_id = c.course_id; SELECT s.student_id, s.student_name, c.course_name FROM students s JOIN courses c ON s.course_id = c.course_id; SELECT student_id, student_name, course_name FROM students LEFT JOIN courses ON students.course_id = courses.course_id;SELECT s.student_id, s.student_name, c.course_name FROM students s RIGHT JOIN courses c ON s.course_id = c.course_id;

This query uses LEFT JOIN to combine records from 'students' and 'courses' tables based on matching 'course_id' values, returning all students even if they are not enrolled in any course.

Q25

Multiple Choice

Which SQL query uses LEFT JOIN to display all transaction details along with their respective client names, including transactions without associated clients?

SQL Code

SELECT t.transaction_id, t.amount, c.client_name
FROM transactions t
LEFT JOIN clients c ON t.client_id = c.client_id;

SELECT t.transaction_id, t.amount, c.client_name FROM transactions t LEFT JOIN clients c ON t.client_id = c.client_id; SELECT t.transaction_id, t.amount, c.client_name FROM transactions t JOIN clients c ON t.client_id = c.client_id; SELECT transaction_id, amount, client_name FROM transactions LEFT JOIN clients ON transactions.client_id = clients.client_id;SELECT t.transaction_id, t.amount, c.client_name FROM transactions t RIGHT JOIN clients c ON t.client_id = c.client_id;

The LEFT JOIN is used to retrieve all transactions along with client names, including transactions that do not have associated clients.

Q26

Multiple Choice

Which SQL query uses LEFT JOIN to combine the 'orders' and 'products' tables to find the total sales amount for each product, including products with no sales?

SQL Code

SELECT p.product_name, SUM(o.quantity * p.price) AS total_sales
FROM products p
LEFT JOIN orders o ON p.product_id = o.product_id
GROUP BY p.product_name;

SELECT p.product_name, SUM(o.quantity * p.price) AS total_sales FROM products p LEFT JOIN orders o ON p.product_id = o.product_id GROUP BY p.product_name; SELECT product_name, SUM(quantity * price) AS total_sales FROM products LEFT JOIN orders ON products.product_id = orders.product_id GROUP BY product_name; SELECT p.product_name, SUM(o.quantity * p.price) AS total_sales FROM products p JOIN orders o ON p.product_id = o.product_id GROUP BY p.product_name;SELECT p.product_name, total_sales = SUM(o.quantity * p.price) FROM products p LEFT JOIN orders o ON p.product_id = o.product_id GROUP BY p.product_name;

The LEFT JOIN is used to combine records from 'orders' and 'products' based on 'product_id', followed by grouping to calculate the total sales for each product, including those with no sales.

Q27

Multiple Choice

Identify the correct SQL query that uses LEFT JOIN to fetch all employees along with their order details, including employees who haven't placed any orders.

SQL Code

SELECT e.employee_id, e.name, o.order_id
FROM employees e
LEFT JOIN orders o ON e.employee_id = o.employee_id;

SELECT e.employee_id, e.name, o.order_id FROM employees e LEFT JOIN orders o ON e.employee_id = o.employee_id; SELECT e.employee_id, e.name, o.order_id FROM employees e JOIN orders o ON e.employee_id = o.employee_id; SELECT employee_id, name, order_id FROM employees LEFT JOIN orders ON employees.employee_id = orders.employee_id;SELECT e.employee_id, e.name, o.order_id FROM employees e RIGHT JOIN orders o ON e.employee_id = o.employee_id;

This query uses LEFT JOIN to combine records from the 'employees' and 'orders' tables where the 'employee_id' matches, showing all employees including those who haven't placed any orders.

Q28

Multiple Choice

Which SQL query uses LEFT JOIN to retrieve all customer orders along with their respective shipping details, including orders without shipping information?

SQL Code

SELECT o.order_id, o.order_date, s.shipping_date
FROM orders o
LEFT JOIN shipping s ON o.order_id = s.order_id;

SELECT o.order_id, o.order_date, s.shipping_date FROM orders o LEFT JOIN shipping s ON o.order_id = s.order_id; SELECT o.order_id, o.order_date, s.shipping_date FROM orders o JOIN shipping s ON o.order_id = s.order_id; SELECT order_id, order_date, shipping_date FROM orders LEFT JOIN shipping ON orders.order_id = shipping.order_id;SELECT o.order_id, o.order_date, s.shipping_date FROM orders o RIGHT JOIN shipping s ON o.order_id = s.order_id;

The LEFT JOIN is used to combine records from the 'orders' and 'shipping' tables based on the common 'order_id', returning all orders including those without shipping details.

Q29

Multiple Choice

Determine the correct SQL query that uses LEFT JOIN to list all teachers along with the classes they are teaching, including teachers who are not assigned to any class.

SQL Code

SELECT t.teacher_id, t.name, c.class_name
FROM teachers t
LEFT JOIN classes c ON t.teacher_id = c.teacher_id;

SELECT t.teacher_id, t.name, c.class_name FROM teachers t LEFT JOIN classes c ON t.teacher_id = c.teacher_id; SELECT teacher_id, name, class_name FROM teachers LEFT JOIN classes ON teachers.teacher_id = classes.teacher_id; SELECT t.teacher_id, t.name, c.class_name FROM teachers t JOIN classes c ON t.teacher_id = c.teacher_id;SELECT t.teacher_id, t.name, c.class_name FROM teachers t RIGHT JOIN classes c ON t.teacher_id = c.teacher_id;

The LEFT JOIN combines records from the 'teachers' and 'classes' tables on the 'teacher_id' column, returning all teachers including those who are not assigned to any class.

Q30

Multiple Choice

Which SQL query correctly uses LEFT JOIN to retrieve a list of all projects and their corresponding manager names, including projects without assigned managers?

SQL Code

SELECT p.project_name, m.manager_name
FROM projects p
LEFT JOIN managers m ON p.manager_id = m.manager_id;

SELECT p.project_name, m.manager_name FROM projects p LEFT JOIN managers m ON p.manager_id = m.manager_id; SELECT project_name, manager_name FROM projects LEFT JOIN managers ON projects.manager_id = managers.manager_id; SELECT p.project_name, m.manager_name FROM projects p JOIN managers m ON p.manager_id = m.manager_id;SELECT p.project_name, m.manager_name FROM projects p RIGHT JOIN managers m ON p.manager_id = m.manager_id;

This query uses LEFT JOIN to combine records from the 'projects' and 'managers' tables based on the 'manager_id' column, returning all projects including those without assigned managers.

MySQL Database Quiz Questions