Fetch current and next row and compare in a loop

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;
}