Scalar Subqueries 

A scalar subquery is a subquery that return exactly one row with exactly one column.

Syntax

<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%.

See

Value expressions, Dynamic Parameter Specification, Query Specification