vhost:

DBDriver mysql
DBDParams "host=127.0.0.1 port=3306 dbname=mydbname user=mydbuser pass=mydbpass"

<Directory /var/www/zeug/dbtest>

       AuthType Basic
       AuthName "Tutorial"
       AuthBasicProvider dbd

       Require valid-user

       AuthDBDUserPWQuery "SELECT password FROM af_users LEFT JOIN af_user_confidential USING (id) WHERE username = '%s'"
</Directory>


-----------
sql-query-commandline-test:
echo "SELECT password FROM af_users LEFT JOIN af_user_confidential USING (id) WHERE username = 'testuser'" | mysql dbhere

output:
password
bc3acb45ab4c5a022f36ac288a54b62d