The modification of data that is already in the database is
referred to as updating. You can update individual rows, all the
rows in a table, or a subset of all rows. Each column can be
updated separately; the other columns are not affected. To
perform an update, you need three pieces of information: The
name of the table and column to update, The new value of the
column, Which row(s) to update. Recall from Chapter 5 that SQL
does not, in general, provide a unique identifier for rows.
Therefore it is not necessarily possible to directly specify
which row to update. Instead, you specify which conditions a row
must meet in order to be updated. Only if you have a primary key
in the table (no matter whether you declared it or not) can you
reliably address individual rows, by choosing a condition that
matches the primary key. Graphical database access tools rely on
this fact to allow you to update rows individually. For example,
this command updates all products that have a price of 5 to have
a price of 10: UPDATE