SET @a = 0;
UPDATE tabellenName SET ID = (@a := @a +1);
ALTER TABLE tabellenName  AUTO_INCREMENT = 1;

$x = 8;
while($x < ($data[7]+1)){
$sql = "insert `tisch1` (`data`) VALUES (0)";
mysqli_query($db,$sql);
$x = $x + 1;		
}