LEFT JOIN preserving all left rows in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all columns from the left table and join with the right table.

SQL

SELECT * FROM employees [1] departments ON employees.department_id = departments.id;

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

AINNER JOIN

BLEFT JOIN

CRIGHT JOIN

DFULL JOIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using INNER JOIN which excludes unmatched rows from the left table.

Using RIGHT JOIN which keeps all rows from the right table instead.

The LEFT JOIN keyword returns all rows from the left table (employees), and the matched rows from the right table (departments). If there is no match, the result is NULL on the right side.

2fill in blank

medium

Complete the code to select employee names and their department names, preserving all employees even if they have no department.

SQL

SELECT employees.name, [1].name AS department_name FROM employees LEFT JOIN departments ON employees.department_id = departments.id;

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

Aprojects

Bemployees

Cmanagers

Ddepartments

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting employees.name as department_name by mistake.

Selecting a table that is not joined in the query.

We want the department name from the departments table, so we select departments.name.

3fill in blank

hard

Fix the error in the join condition to correctly join employees with departments preserving all employees.

SQL

SELECT employees.name, departments.name FROM employees LEFT JOIN departments ON employees.[1] = departments.id;

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

Adepartment_id

Bmanager_id

Cdept_id

Did

Attempts:

3 left

💡 Hint

Common Mistakes

Using employees.id which is the employee's own id, not the department reference.

Using a column that does not exist in employees.

The join condition must match employees.department_id with departments.id to link employees to their departments.

4fill in blank

hard

Fill both blanks to select employee names and their department names, showing 'No Department' if the department is missing.

SQL

SELECT employees.name, COALESCE(departments.[1], [2]) AS department_name FROM employees LEFT JOIN departments ON employees.department_id = departments.id;

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

Aname

B'No Department'

C'Unknown'

Did

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong column name from departments.

Not providing a string literal for the default value.

We use departments.name to get the department name. If it is NULL, COALESCE returns the string 'No Department' instead.

5fill in blank

hard

Fill all three blanks to count employees per department, including departments with zero employees.

SQL

SELECT departments.[1], COUNT(employees.[2]) AS employee_count FROM departments LEFT JOIN employees ON employees.[3] = departments.id GROUP BY departments.name;

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

Aname

Bid

Cdepartment_id

Attempts:

3 left

💡 Hint

Common Mistakes

Counting department id instead of employee id.

Joining on wrong columns causing incorrect counts.

We select departments.name to show the department name. We count employees.id to count employees. The join condition matches employees.department_id to departments.id.