Why advanced joins matter in SQL - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all columns from the employees table.

SQL

SELECT [1] FROM employees;

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A*

Ball

Ccolumns

Deverything

Attempts:

3 left

💡 Hint

Common Mistakes

Using words like 'all' or 'columns' instead of *

Leaving the SELECT statement incomplete

The asterisk (*) selects all columns from the table.

2fill in blank

medium

Complete the code to join employees with departments on department_id.

SQL

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

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AFULL

BLEFT

CRIGHT

DINNER

Attempts:

3 left

💡 Hint

Common Mistakes

Using LEFT JOIN when only matching rows are needed

Using FULL JOIN which includes unmatched rows

INNER JOIN returns rows when there is a match in both tables.

3fill in blank

hard

Fix the error in the join condition to correctly join orders and customers.

SQL

SELECT orders.id, customers.name FROM orders INNER JOIN customers ON orders.customer_id [1] customers.id;

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A=

B==

C!=

D<>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '='

Using '!=' or '<>' which mean not equal

In SQL, the single equals sign (=) is used for equality comparison in JOIN conditions.

4fill in blank

hard

Fill both blanks to select employee names and their manager names using a self join.

SQL

SELECT e.name AS employee, m.name AS manager FROM employees e [1] JOIN employees m ON e.[2] = m.id;

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ALEFT

BRIGHT

Cmanager_id

Demployee_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using RIGHT JOIN which excludes some employees

Using wrong column like 'employee_id' for the join

LEFT JOIN includes all employees even if they don't have a manager. The join condition uses 'manager_id' to link employees to their managers.

5fill in blank

hard

Fill all three blanks to select product names, order quantities, and customer names using multiple joins.

SQL

SELECT p.[1], o.[2], c.[3] FROM orders o INNER JOIN products p ON o.product_id = p.id INNER JOIN customers c ON o.customer_id = c.id;

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Aname

Bquantity

Dprice

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting price instead of name for products

Mixing up column names between tables

We select product name (p.name), order quantity (o.quantity), and customer name (c.name) to get meaningful order details.