Better defined like this view sds select distinct sname

Chapter 9

S Q L a n d V i e w s

 A view is a derived relvar; in other words, it’s a relvar that’s explicitly derived (and known to be derived, at least by some people) from certain other relvars. Note: If you’re wondering what the difference is between a derived relvar and a virtual one (see the previous bullet item), I should explain that all virtual relvars are derived but some derived ones aren’t virtual. See the section “Views and Snapshots” later in this chapter.

 A view is a “window into” the relvars from which it’s derived; thus, operations on the view are to be understood as “really” being operations on those underlying relvars.

VIEWS ARE RELVARS

Of those informal characterizations listed above of what a view is, the following definition might appear to favor one over the rest─but those informal characterizations are all equivalent anyway, loosely speaking:

Note that these are restriction views─their value at any given time is a certain restriction of the value at that time of relvar S. Some syntax issues:

 The parentheses in the SQL examples are unnecessary but not wrong; I include them for clarity. The parentheses in the Tutorial D examples are required.

Note in particular that this latter style means we’re telling the system once instead of twice that one of the view columns is called SNAME.

 CREATE VIEW in SQL also allows WITH CHECK OPTION to be specified if (but only if!) it regards the view as updatable. Recommendation: Always specify this option if possible. See the section “Update Operations” for further discussion.