Updates

1. Updating allows us to change some values in a tuple without necessarily changing all.
2. Some examples:
   1. To increase all balances by 5 percent.

      

      This statement is applied to every tuple in deposit.

   2. To make two different rates of interest payment, depending on balance amount:

      

      Note: in this example the order of the two operations is important. (Why?)

3. In general, where clause of update statement may contain any construct legal in a where clause of a select statement (including nesting).
4. As before, any select embedded must not reference the relation being updated.

   For example, the following is an ambiguous request:

   

   The result would depend on the order in which tuples are processed.