States a condition.
|expression||A logic expression using conditional and Boolean operators.|
WHERE states the conditions that restrict processing in the main command to only certain members of the set specification, contingent on a value within each of the (related) records being considered.
Of all the available (related) records in the record-containing objects, only those records for which the condition is logically true become members of the set.
list all Employees where LastName = Smith and FirstName = J?
The preceding command lists only those employees whose last name is Smith and whose first name starts with the letter J.