SELECT max(logs) as logs,id,'table1' type
FROM table1 WHERE message NOT LIKE "[ SYSTEM%"
UNION
SELECT max(logs) as logs,'table2' type
FROM table2 WHERE message LIKE "[ SYSTEM%"
;

按字段类型,您可以了解从哪个表接收数据.
但如果你的解释不好,你应该使用单独的查询：

<?PHP

$dbh = new PDO('MysqL:dbname=test;host=127.0.0.1','root');

$sth = $dbh->prepare('
    SELECT max(logs) as logs,id
    FROM table1 WHERE message NOT LIKE :like
');
$sth->bindParam(':like',$like,PDO::PARAM_STR);
$sth->execute();
$result = $sth->fetchAll(PDO::FETCH_ASSOC);

if (empty($result)) {
    $sth = $dbh->prepare('
        SELECT max(logs) as logs,id
        FROM table2 WHERE message LIKE :like
    ');
    $sth->bindParam(':like',PDO::PARAM_STR);
    $sth->execute();
    $result = $sth->fetchAll(PDO::FETCH_ASSOC);
}

var_export($result);