Relational algebra - List the names of employees who do not work in the project 1

Question

I've the following tables:

Employee
|name|employee_cod|

Project
|name|project_cod|

Work
|employee_cod|project_cod|

So, how can I list the names of employees who not work, for example, in the project with project_cod = 1, with relational algebra?

The following not work:

π(employee.name(σ work.project_cod != 1 (Employee ∞ Work ∞ Project)

Because if I have the following data in work table:

| employee_cod | project_cod |
-------1--------------1-------
-------1--------------2-------
-------1--------------3-------
-------2--------------2-------

This σ work.project_cod != 1 will result in:

| employee_cod | project_cod |
-------1--------------2-------
-------1--------------3-------
-------2--------------2-------

But the employee with code = 1 should not be returned, because it participates in project 1

Answer 1

You first find all the employees who do work on the project. Those who don't are produced by relational difference (minus) operator.

Answer 2

Thank you for the tip Tegiri.
The solution is:

π name (Employee) - ( π name (σ project_cod = 1 (Employee ∞ Work)))