This construct allows you to update the contents of a field. If you wish to change the contents of one or more fields, you need to first narrow in on just the field or fields to be changed, in much the same way you use the SELECT command. Consider the following example for working of update clause.
UPDATE classics SET author='Mark Twain (Samuel Langhorne Clemens)' WHERE author='Mark Twain'; UPDATE classics SET category='Classic Fiction' WHERE category='Fiction';
In the first query Mark Twain’s real name of Samuel Langhorne Clemens was appended to his pen name in brackets, which affected only one row. The second query, however, affected three rows, because it changed all occurrences of the word Fiction in the category column to the term Classic Fiction.