The WHERE clause restricts a result set to those rows which satisfy the predicate in this clause. Let us suppose that there are such conformities between a memory size of the personal computer and a type of used operational system (conditionally):
If we shall want to choose computers in accordance with the type of OS (note that such column isn't in PC table) it is possible to write the following query:
Here we choose the models fit to the operational system W98. The result of the execution of this query is given below.
It can appear more useful than it seems at first sight if you'll keep in mind confidentiality of the information. For example, on the client side it is possible to issue queries which will operate with such categories as high-, mid-, or low-paid expert. Real criteria (salaries) will be hidden, say, in stored procedure which will take something like character string "average" as an input parameter.