I am currently studying for an exam in relational algebra and have come across the following: Please note that all italic words are replacing the symbols that represent those words.

Schema:
EMPLOYEE: Number, Name, Age, Salary
SUPERVISION: Head, Employee

Find the reg numbers of the supervisors of the employees earning more than 40k

Solution:

project Head(SUPERVISION join EMPLOYEE = Number(select salary > '40k' (EMPLOYEE)))

Before I seen the solution I had come up with the following:

R = project Number (select salary > '40k' (EMPLOYEE))
project Head(Supervision join R)

Can someone tell me if my original solution would be considered correct? If not, why does it differ from the given solution? This algebra is doing my head in!

Where is the information "the supervisors of the employees"?

Sorry the supervisors are also employees should have specified that

Please update the following informations according to your statement.
I'm not so well at English, so I couldn't ubderstand well the phrase "should have specified that".

And, it is not clear by what condition(s) to "Supervision join R".
There is no common column.

Relationship Schema:

Employee: Number, Name, Age, Salary
Supervision: Supervisor Number, Employee Number

Is this better?