For this exercise I've created a db fiddle you can interact with, that comes with a table and data. Feel free to use it so you don't have to set up your table and data yourself!
Select all user data for those users who earned a salary greater than 60000.
Select the surname of those users who earned a salary greater than 80000 or are over 50 years old.
Select all user data for those users who either:
Earned a salary greater than 35000 and are under 20 years old.
Or earned a salary greater than 80000 and are under 30 years old.
Here's the solution for exercise 1:
SELECT * FROM users WHERE salary > 60000;
Here's the solution for exercise 2:
SELECT surname FROM users WHERE salary > 80000 OR age > 50;
Here's the solution for exercise 3:
SELECT * FROM users WHERE (salary > 35000 AND age < 20) OR (salary > 80000 AND age < 30);