A scalar subquery is a subquery that return exactly one row with exactly one column.
Syntax
<scalar subquery> ::= '('<query specification>')'.
Syntax
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%.