Question

I’m using MySQL with PHP. This is like my table: (I’m using 3 values, but there are more)

id | 1 | 2 | 3
---+---+---+----
1  | 3 |12 |-29
2  | 5 |8  |8
3  | 99|7  |NULL

I need to get the greatest value’s column name in a certain row. It should get:

id | maxcol
---+-------
1  |   2
2  |   2
3  |   1

Are there any queries that will do this? I’ve been trying, but I can’t get it to work right.

Answer 1

Are you looking for something like the GREATEST function? For example:

SELECT id, GREATEST(col1, col2, col3)
    FROM tbl
    WHERE ...

Combine it with a CASE statement to get column names:

SELECT id, CASE GREATEST(COALESCE(`1`, -2147483646), COALESCE(`2`, -2147483646), COALESCE(`3`, -2147483646))
         WHEN `1` THEN 1
         WHEN `2` THEN 2
         WHEN `3` THEN 3
         ELSE 0
      END AS maxcol
    FROM tbl
    WHERE ...

It’s not pretty. You’d do better to follow Bill Karwin’s suggestion and normalize, or simply take care of this in PHP.

function findcol($cmp, $arr, $cols=Null) {
   if (is_null($cols)) {
      $cols = array_keys($arr);
   }
   $name = array_shift($cols);
   foreach ($cols as $col) {
       if (call_user_func($cmp, $arr[$name], $arr[$col])) {
           $name = $col;
       }
   }
   return $name;
}

function maxcol($arr, $cols=Null) {
   return findcol(create_function('$a, $b', 'return $a < $b;'), $arr, $cols);
}