A WHERE clause is an optional part of a SelectExpression, DELETE
statement, or UPDATE statement. The WHERE clause lets you select
rows based on a boolean expression. Only rows for which the
expression evaluates to TRUE are returned in the result, or, in
the case of a DELETE statement, deleted, or, in the case of an
UPDATE statement, updated. WHERE Boolean expression Boolean
expressions are allowed in the WHERE clause. Most of the general
expressions listed in Table of general expressions, can result
in a boolean value. In addition, there are the more common
boolean expressions. Specific boolean operators listed in Table
10, take one or more operands; the expressions return a boolean
value. -- find the flights where no business-class seats have --
been booked SELECT * FROM FlightAvailability WHERE
business_seats_taken IS NULL OR business_seats_taken = 0 -- Join
the EMP_ACT and EMPLOYEE tables -- select all the columns from
the EMP_ACT table and -- add the employee's surname (LASTNAME)
from the EMPLOYEE table -