Updating through Expressions - View Updating and Relational Theory - page 203

case that U is defined for the other one as well, and it has the same effect on both. Indeed, I'm
on record 1 as making a series of rather dogmatic assertions on such matters, all of them along the
following lines:
The “Semantics Not Syntax Principle”: The semantics of view updating should not
depend on the particular syntactic form in which the view definition in question happens to
be expressed.
Note: Before I try to elaborate on this “principle”—if principle it truly is—I need to clarify
a couple of points. First, with respect to queries, what I said before, in essence, was this: If Q is
a query on V1 , then that same query Q is defined for V2 as well, and it produces the same result
on both. But Q obviously can't quite be “the same query” for both views, because of course the
views have different names. Thus, for example, given views V1 and V2 as defined above for
Example 1, if I claim that the query
V WHERE STATUS > 15
is defined for both V1 and V2, what I mean is that the symbol V can sensibly be replaced in that
query by either V1 or V2. That's what I meant when I said the expression Q can be said to
represent “the same query” for both views. And given our usual sample values, that query will
of course return the same result in both cases:
┌─────┬───────┬────────┬────────┐
│ SNO │ SNAME │ STATUS │ CITY │
├═════┼───────┼────────┼────────┤
│ S1 │ Smith │ 20 │ London │
│ S3 │ Blake │ 30 │ Paris │
│ S4 │ Clark │ 20 │ London │
└─────┴───────┴────────┴────────┘
And when I use the phrase “the same update,” of course, I mean it to be understood in the same
kind of way.
The second point I need to clarify has to do with what it means, precisely, for two views to
be “equivalent.” Of course, I've appealed to an intuitive understanding of this notion a couple of
times already in this discussion, first in my investigation into Example 1, and then in my attempt
to draw a general conclusion from that investigation. But the question is: Is that intuitive
understanding sufficient? As you've probably guessed already, the answer to this question is no .
In fact, much of the rest of this chapter consists of an attempt to pin down much more precisely
just what such a notion—i.e., of equivalence of views—might really mean.
Back, then, to the “semantics not syntax principle.” As I've indicated, I assumed for a long
time that this “principle” obviously made sense. (After all, it certainly makes sense in another
1 E.g., in “View Updating,” Appendix E of Databases, Types, and the Relational Model: The Third Manifesto , by Hugh Darwen
and myself (3rd edition, Addison-Wesley, 2006) and elsewhere.