Installation Troubleshooing - DB Error no database selected

PROBLEM: I'm getting this when I logon to Drupal with civiCRM installed: DB Error: no database selected.

RESPONSE: Check your civicrm.settings.php
Make sure you have the right value set for CIVICRM_DSN.

FULL ERROR REPORT:

Error Details:

Array
(
    \[callback\] => Array
        (
            \[0\] => CRM_Core_Error
            \[1\] => handle
        )

    \[code\] => -14
    \[message\] => DB Error: no database selected
    \[mode\] => 16
    \[debug_info\] => SELECT * 
 FROM civicrm_uf_match 
 
 WHERE (  civicrm_uf_match.uf_id = 1 )  AND (  civicrm_uf_match.domain_id = 1 ) 
 
 
 
 \[nativecode=** \]
    \[type\] => DB_Error
    \[user_info\] => SELECT * 
 FROM civicrm_uf_match 
 
 WHERE (  civicrm_uf_match.uf_id = 1 )  AND (  civicrm_uf_match.domain_id = 1 ) 
 
 
 
 \[nativecode=** \]
    \[to_string\] => \[db_error: message="DB Error: no database selected" code=-14  mode=callback callback=CRM_Core_Error::handle prefix="" info="SELECT * 
 FROM civicrm_uf_match 
 
 WHERE (  civicrm_uf_match.uf_id = 1 )  AND (  civicrm_uf_match.domain_id = 1 )

[Source. ]