Shell Programming and Scripting

<?php

for($i=2;$i<2749750;$i++) {
	//Get the previous row
	$sql_prev = "SELECT * FROM trace WHERE ID=".($i-1);
        $result_prev = mysql_query($sql_prev);
	$row_prev = mysql_fetch_assoc($result_prev);
	//Get the current row
	$sql_cur = "SELECT * FROM trace WHERE ID=".$i;
	$result_cur = mysql_query($sql_cur);
	$row_cur = mysql_fetch_assoc($result_cur);
	
        //Now select the corresponding ASN numbers from the other table
	$sql_prev_asn = "SELECT * FROM asn_number WHERE ip_address='$row_prev[ip]'";
	$result_prev_asn = mysql_query($sql_prev_asn);
	$row_prev_asn = mysql_fetch_assoc($result_prev_asn);

	$sql_cur_asn = "SELECT * FROM asn_number WHERE ip_address='$row_cur[ip]'";
	$result_cur_asn = mysql_query($sql_cur_asn);
	$row_cur_asn = mysql_fetch_assoc($result_cur_asn);
		
	if(($row_prev_asn['asn'] != $row_cur_asn['asn'])) {
		$sql = "INSERT INTO br(asn1,ip1,asn2,ip2) VALUES('$row_prev_asn[asn]','$row_prev[ip]','$row_cur_asn[asn]','$row_cur[ip]')";
		$result = mysql_query($sql) or die(mysql_error());

	}
}

?>

select * from table;

select specific_column from table;

i=2; i<somenumber; i++

i=1 ( executed )
i=2 ( executed )

i=2 ( executed )  //why this should be executed again, value is known in the previous iteration itself ???
i=3 ( executed )

select ip from trace where trace_id >= $i and trace_id <= $i+1;

select column_with_running_numbers_sequence, ip from trace;

insert
autocommit
insert
autocommit
insert
autocommit

insert
insert
insert
explicit commit

SELECT id, ip FROM trace;

while($rows = mysql_fetch_assoc($result) {
//Perform operations on rows. But how will I get the previous row or next row in this loop?
}