$db->quoteName(mixed $name, mixed $as = null)
With the quoteName-function you can quote column names. It accepts strings and arrays. A simple quote with alias (the AS) would look like:
$as (the second parameter) is always optional. So if you don't want to give your column an alias just write
With an array it would like this:
$db->quoteName(array('ju.email', 'ju.lastvisitDate'), array(null, 'last_login'))
Note the null value in the second array if you don't want to give a column an alias.
As long as you don't use any reserved keywords or variables (e.g. submitted by the user) as column names I would not escape them. See also this: