Scalar Subqueries
A scalar subquery is a subquery that return exactly one row with exactly one column.
<scalar subquery> ::= '('<query specification>')'.
Example
UPDATE employees
SET salary = salary * 1.1
WHERE 2 * salary <
( SELECT MAX( salary ) FROM employees ) )
Comparison Predicate and Scalar Subquery. The salary of all employees that earn less than half as much as the best paid employee is raised by 10%.