!- Converted with LaTeX2HTML 0.6.4 (Tues Aug 30 1994) by Nikos Drakos (email@example.com), CBLU, University of Leeds ->
Let the view loan-info be given to the clerk:
This insertion is represented by an insertion into the actual relation borrow, from which the view is constructed.
However, we have no value for amount. A suitable response would be
The symbol null represents a null or place-holder value. It says the value is unknown or does not exist.
This view lists the cities in which the borrowers of each branch live.
Now consider the insertion
Using nulls is the only possible way to do this (see Figure 3.22 in the textbook).
If we do this insertion with nulls, now consider the expression the view actually corresponds to:
As comparisons involving nulls are always false, this query misses the inserted tuple.
To understand why, think about the tuples that got inserted into borrow and customer. Then think about how the view is recomputed for the above query.