How do you add a prefix to an existing drupal database for single site use?
If you have phpMyAdmin the simplest way is to:


go to the "Structure" tab (where there is the list of tables).
select all the tables (with the "Check All" checkbox at the end of the list)
in the dropdown list close to the "Check All" select the action "Add table prefix" (or "Replace table prefix" if that is the case)
insert the prefix, press the button and let it do all the work.


After that remember to specify the prefix in the site\default\settings.php

array (
  'database' =&gt; 'sample_drupal_db',
  'username' =&gt; 'sample_drupal_usr',
  'password' =&gt; 'sample_drupal_pwd',
  'host' =&gt; 'localhost',
  'port' =&gt; '',
  'driver' =&gt; 'mysql',
  'prefix' =&gt; 'prefix_', // &lt;&lt;== prefix here
),