A predicate is a logical operator that evaluates to a Boolean value—true or false. Many people think of a predicate as the
WHERE clause in a SQL statement, but that is only one particular case. In SQL, you can write the following:
SELECT name, address FROM employee WHERE department = 'research';
department = 'research' serves as the predicate. For each record that might be returned by the
department = 'research' is evaluated based on that record’s data and the result is either true or false. The records where the result is TRUE are returned ...