Update of a view

1. The view update anomaly previously mentioned in Chapter 3 exists also in SQL.
2. An example will illustrate: consider a clerk who needs to see all information in the loan relation except amount.

   Let the view branch-loan be given to the clerk:

    create view branch-loan as
   
   		 		 select bname, loan#
   
   		 		 from loan
   
   

   Since SQL allows a view name to appear anywhere a relation name may appear, the clerk can write:

    insert into branch-loan
   
   		 		 values (``SFU'', ``L-307'')
   
   

   This insertion is represented by an insertion into the actual relation loan, from which the view is constructed. However, we have no value for amount.

   This insertion results in (``SFU'', ``L-307'', null) being inserted into the loan relation.

   As we saw, when a view is defined in terms of several relations, serious problems can result. As a result, many SQL-based systems impose the constraint that a modification is permitted through a view only if the view in question is defined in terms of one relation in the database.