White SQL queries for below questions:

$1.$ Find all the employees with salary greater than $10,000$

$2.$ Find all the departments where total number of employees are greater than $5.$

$3.$ Find a department with maximum number of projects.

$4.$ Find employees currently working on more than one projects.

$5.$ Find all the projects where salary expenditure is greater than budget of the project.

Answer below questions:

There should not be duplicate department names. In that case, what we have to do$?$

$2.$ How to make the query in question $1)$ as fast as possible?