Any ways to create count column like mysql "COUNT(*) as counted" Column with d7's db_select()?
This error simply means that column does not exist in your database which means u have to write addExpression() method for solving these problem as follows and even you have not joined the table properly use your query as below

$query = db_select('users','u');
$query = $query-&gt;extend('PagerDefault')-&gt;extend('TableSort');
$query-&gt;leftJoin('mycustom_report','r','r.uid=u.uid');
$query-&gt;fields('r');
$query-&gt;fields('u');
$query-&gt;addExpression('COUNT(uid)', 'uid_count');
$result = $query-&gt;execute()-&gt;fetchAll();


this will surely removes your error the main issue with your query is that you are combining the db_query syntax with db_select