Exists Predicate
An exists predicate takes the form:
EXISTS (subquery)
An exists predicate expression evaluates to true if the set represented by subquery is non‑empty. For example:
select ename
from employee
where exists
(select *
from dept
where dno = employee.dept
and floor = 3);
It is typical, but not required, for the subquery argument to exists to be of the form select *.