MySQL Update by Row number - mysql

Question

I need to update rows by their number(not AI ID, cause some of the rows may will be removed). How can I do this?
I mean something like this:
UPDATE cars SET idx = value WHERE row_number = i
I would do this in a 'for' statement, and i is the integer of my statement. So I would update every row in the statement.
Sorry for my bad english, and thanks!

Answer 1

Here's a pure MySQL solution:
/*test data*/
create table foo (id int auto_increment primary key, a int);
insert into foo (a) values (10), (11), (12);
/*update statement*/
update foo
set a = 5
where id = (
select id from (
select id, #rownum:=#rownum + 1 as rownumber
from foo, (select #rownum:=0) vars order by id
) sq where rownumber = 2
);
Results in:
| ID | A |
-----|----|--
| 1 | 10 |
| 2 | 5 |
| 3 | 12 |
Feel free to ask if you have any questions about this.
Also, note the order by id in there. It's important, cause in a database there is no first or last row. Without an order by clause theoretically there could be each time a different result.
You can also see it working live here in an sqlfiddle.

Answer 2

i don't know this about mysql but you can do this in php
$row_number=? ;//the row no of mysql you want to change the id
$id=? ;//the new id
mysql_connect //do it yourself
$query="select 8 from tablename"; //the query
$result=mysql_query($qyery,$conn);
$count=0;
while($row=mysql_fetch_array($result)) // fetch each row one by one an put data in array $row
{
$count++; //increment count means the no of rows are incermented
if($count==$rownumber) //the row where you want to edit the id
{
$query1="update tablename set id='".$id."' where id=".$row["id"]; //new query on that particular row
$result1=mysql_query($query1,$conn);
}
}
this will work , just modify this code according to your use