Tuple Relational Calculus in a DBMS

[momentum]

<Moderator's note: Moved from a technical forum and thus no template.>

I'm solving a problem using tuple relational calculus ( TRC) in DBMS.

Problem
Find the name of all the employees who work for XYZ Bank Corporation.

Solution ( as given in my book)

I don't understand why book is using S . We have t . Since both of them represents tuples , we could just use t in place of S and get moving.

Can anyone please explain why we required S ?

 

[willem2]

t is a tuple that contains only the name of an employee, so it is not a member of WORKS. I think the exporesson for the query should start with {t : {emp_name} | ∃S ∈ WORKS ......

 

[momentum]

t is a tuple that contains only the name of an employee, so it is not a member of WORKS. I think the exporesson for the query should start with {t : {emp_name} | ∃S ∈ WORKS ......

It returns t or S ?

 

[willem2]

It returns t or S ?

The entire expression {t: {emp_name} | ∃ S ∈ WORKS ( ...... ) } defines a relation that is a set of tuples with just one attribute. (only the names of the employess of XYZ). t is used, because we only want the employee names, not the comp_names

 

[momentum]

The entire expression {t: {emp_name} | ∃ S ∈ WORKS ( ...... ) } defines a relation that is a set of tuples with just one attribute. (only the names of the employess of XYZ). t is used, because we only want the employee names, not the comp_names

okay ...my confusion here is ,
we call t as tuple.
do we also call S as tuple here ? Yes/No ?

 

[willem2]

okay ...my confusion here is ,
we call t as tuple.
do we also call S as tuple here ? Yes/No ?

S is an element of the WORKS relationship, so it must be a tuple.