Why joins are essential in PostgreSQL - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all columns from two tables joined on a common column.

PostgreSQL

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

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

AJOIN

BSELECT

CWHERE

DGROUP BY

Attempts:

3 left

💡 Hint

Common Mistakes

Using SELECT instead of JOIN causes syntax errors.

Using WHERE without JOIN does not combine tables properly.

The JOIN keyword combines rows from two tables based on a related column.

2fill in blank

medium

Complete the code to get employee names and their department names using a join.

PostgreSQL

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

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

Aemployees

Bprojects

Cmanagers

Ddepartments

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting from the wrong table causes errors or wrong results.

Using a table not joined in the query causes errors.

We select the department name from the departments table.

3fill in blank

hard

Fix the error in the join clause to correctly join employees and departments.

PostgreSQL

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

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

Aid

Bdepartment_id

Cname

Ddept_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong column name causes join errors.

Mixing up primary and foreign keys leads to wrong results.

The department_id in employees matches the id in departments.

4fill in blank

hard

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

PostgreSQL

SELECT employees.[1], projects.[2] FROM employees JOIN projects ON employees.project_id = projects.id;

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

Aname

Bid

Ctitle

Ddepartment_id

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting wrong columns leads to confusing results.

Using IDs instead of names or titles does not show meaningful data.

We select name from employees and title from projects.

5fill in blank

hard

Fill all three blanks to select employee names, department names, and project titles using joins.

PostgreSQL

SELECT employees.[1], departments.[2], projects.[3] FROM employees JOIN departments ON employees.department_id = departments.id JOIN projects ON employees.project_id = projects.id;

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

Aname

Btitle

Did

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up column names causes wrong output.

Using 'id' instead of descriptive columns shows numbers, not names.

Select employee name, department name, and project title.