Fetch current and next row and compare in a loop - php

Question

I would like to compare two values from a table:
This is my table:
ID;VALUE
1;700
2;600
3;800
4;900
This is my query:
$stmt = $db->query("SELECT ID, VALUE FROM TABLE);
Now i like to compare the result of the current row with the next row. In mysql it was easy beacause i have to set the row number. I did not find any solution with PDO.
This is my code yet:
while($row = $stmt->fetch(PDO::FETCH_ASSOC)) {
$id = $row["ID"];
$val_current = $row["VALUE"];
$row_next = $stmt->fetch(PDO::FETCH_ASSOC);
$val_next = $row["VALUE"];
if ($val_current > $val_next){
echo "$id is greate!";
}else{
echo "$id is less!";
}
}
The Result is:
1 is greater
3 is less
He missed to check the ID 2 agains ID 3 becaue i fetch the next element in the while loop. So i fetch it to get the next value and i fetch again in the loop. Can i reposition the cursor at the end of the loop?

Answer 1

If you are running MySQL 8.0, this is straight-forward with window function lead():
select
t.*,
(value - lead(value) over(order by id) > 0) is_greater
from mytable t
This gives you a boolean flag called is_greater, with following possible values:
1: this value is greater than the next one
0: this value is smaller than the next one
null: there is no next value
Demo on DB Fiddle:
ID | VALUE | is_greater
-: | ----: | ---------:
1 | 700 | 1
2 | 600 | 0
3 | 800 | 0
4 | 900 | null
In earlier versions, one option is to use a correlated subquery:
select
t.*,
(value - (
select t1.value
from mytable t1
where t1.id > t.id
order by t1.id limit 1
) > 0) is_greater
from mytable t

Answer 2

You just need to remember the previous row and use it in the next iteration pretending it's the current one, whereas the current row will serve as the next one.
$row = null;
while($row_next = $stmt->fetch(PDO::FETCH_ASSOC)) {
if ($row !== null)) {
$id = $row["ID"];
$val_current = $row["VALUE"];
$val_next = $row_next["VALUE"];
if ($val_current > $val_next){
echo "$id is greater!";
}else{
echo "$id is less!";
}
echo "\n";
}
$row = $row_next;
}