consultant => 000010, read => 0
consultant => 000010, read => 0
consultant => 000010, read => 0
consultant => 000010, read => 1
consultant => 000011, read => 1
consultant => 000019, read => 0
consultant => 000034, read => 1

$query = $dbh->query("SELECT DISTINCT consultant FROM Messages;");

consultant => 000010 (3)
consultant => 000019 (1)

000010 => 3
000019 => 1

$consultant = $query->fetchAll();					
if (count($consultant) > 0) {
    foreach ($consultant as $row) {
        ... тут я тоже напишу код
    }
}

$query = $dbh->query("SELECT DISTINCT consultant FROM Messages WHERE read = '0';");

consultant => 000010, read => 0
consultant => 000010, read => 0
consultant => 000010, read => 0
consultant => 000010, read => 1
consultant => 000011, read => 1
consultant => 000019, read => 0
consultant => 000034, read => 1

SELECT T1.consultant, T1.count, T2.unread
FROM
  (SELECT consultant, COUNT(consultant) AS count
   FROM Messages GROUP BY consultant) T1,
   
  (SELECT consultant, COUNT(consultant) AS unread
   FROM Messages WHERE read = '0' GROUP BY consultant) T2
WHERE T1.consultant = T2.consultant;

consultant => 000010, count => 4, unread => 3
consultant => 000019, count => 1, unread => 1

consultant => 000010, count => 4, unread => 3
consultant => 000011, count => 1, unread => 0
consultant => 000019, count => 1, unread => 1
consultant => 000034, count => 1, unread => 0

SELECT 
    T1.consultant, count(T1.consultant) AS count, T2.unread
FROM 
    Messages T1
LEFT JOIN 
    (SELECT consultant, count(*) AS unread  FROM Messages WHERE read = '0' GROUP BY consultant) T2
ON 
    T1.consultant = T2.consultant
GROUP BY 
    T1.consultant