Why joins are needed 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

Cemployees

Dcolumns

Attempts:

3 left

💡 Hint

Common Mistakes

Using the table name instead of *

Using ALL which is not valid in SELECT

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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ALEFT

BRIGHT

COUTER

DINNER

Attempts:

3 left

💡 Hint

Common Mistakes

Using OUTER JOIN without specifying LEFT or RIGHT

Using LEFT 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.[1] = customers.id;

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Aorder_id

Bid

Ccustomer_id

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using orders.id which is the order's own id

Using customers.name which is not an id

The orders table has a foreign key customer_id that matches customers.id.

4fill in blank

hard

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

SQL

SELECT employees.name, departments.[1] FROM employees [2] JOIN departments ON employees.department_id = departments.id;

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

Aname

Bid

CINNER

DLEFT

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting department id instead of name

Using LEFT JOIN which includes unmatched employees

We select the department's name column and use INNER JOIN to get matching rows.

5fill in blank

hard

Fill all three blanks to select order id, customer name, and order date with a join.

SQL

SELECT orders.[1], customers.[2], orders.[3] FROM orders INNER JOIN customers ON orders.customer_id = customers.id;

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

Aid

Bname

Corder_date

Dcustomer_id

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting customer_id instead of order_date

Mixing up column names between tables

We select order id, customer name, and order date columns to get full order info.