SQL equivalent of relational algebra DIVISION

Question

Here is an example T(A) = RENTED(A,C) / BOATS(C)

select distinct R1.A from RENTED R1  
where not exists                     
  (select * from SAILBOAT S     
   where not exists                  
     (select * from RENTED R2        
      where R1.A = R2.A              
        and R2.C = S.C)              
   );

My question is, if NOT EXISTS just returns TRUE or FALSE, how does SELECT distinct R1.A know which values to return?

For example this jsfiddle

This query returns EVERYTHING in the numbers column if there exists a number = 5

Answer 1

As wildplasser and sqlvogel have mentioned, the subquery gets executed once for each row in the outer query.

The result of the subquery (TRUE / FALSE) determines whether the row in the outer query would be returned. Invariably, the parent key (identifier) columns of the outer query would be referenced within the subquery to check its existence in other tables. This reference makes the subquery a "correlated subquery".

Please see the updated fiddle.