Databases Reference
In-Depth Information
into
V
causes
t
be inserted into both
A
and
B
, we're effectively treating logical OR as logical
AND once again.
3
We also saw that the foregoing criticism applied to the first version of the “parts on sale”
vs. “parts in stock” example. However, the real problem with that example was that, given a
particular tuple to be inserted into the union ULK, the DBMS was unable to tell whether that
tuple logically belonged in relvar PL or relvar PK or both. In effect, the pertinent restriction
condition for each of PL and PK in that example was simply TRUE (contrast the situation with
Example 2, the NLS vs. NPS example, as discussed earlier in the chapter). And my proposed
solution to this problem was, in effect, to redesign the database in such a way that the DBMS
could tell which relvar(s) a given tuple logically belonged in after all—a solution that I venture
to suggest will often work in practice, and indeed is likely to offer benefits in other areas as well,
in addition to view updating as such.
3
I remind you again that David McGoveran has a proposal for resolving this ambiguity, which I'll be discussing in Chapter 15.
