Why PostgreSQL advanced features matter - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

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

PostgreSQL

SELECT [1] FROM employees;

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

A*

Ball

Ceverything

Dcolumns

Attempts:

3 left

💡 Hint

Common Mistakes

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

Leaving the SELECT statement incomplete

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

2fill in blank

medium

Complete the code to filter rows where the salary is greater than 50000.

PostgreSQL

SELECT * FROM employees WHERE salary [1] 50000;

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

A=

B>

C<

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' which selects only salaries exactly 50000

Using '<' which selects salaries less than 50000

The greater than symbol (>) filters salaries above 50000.

3fill in blank

hard

Fix the error in the query to count the number of employees.

PostgreSQL

SELECT COUNT([1]) FROM employees;

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

Aemployee_id

Bsalary

Cname

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Counting a specific column which may miss rows with NULL values

Leaving COUNT empty or with invalid syntax

Using COUNT(*) counts all rows regardless of column values.

4fill in blank

hard

Fill both blanks to create a dictionary of employee names and their salaries where salary is above 60000.

PostgreSQL

SELECT jsonb_object_agg([1], [2]) FROM employees WHERE salary > 60000;

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

Aname

Bsalary

Cemployee_id

Ddepartment

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping keys and values

Using columns not relevant to the dictionary

jsonb_object_agg(key, value) creates a JSON object with keys and values. Here, names are keys and salaries are values.

5fill in blank

hard

Fill all three blanks to create a JSON object with employee names as keys, their salaries as values, but only include those in the 'Sales' department.

PostgreSQL

SELECT jsonb_object_agg([1], [2]) FROM employees WHERE department = '[3]';

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

Aname

Bsalary

CSales

DMarketing

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong department name

Swapping keys and values in jsonb_object_agg

This query creates a JSON object mapping employee names to salaries, filtered to only those in the Sales department.