MySQL: Auto increment temporary column in select statement

This will give you a consecutive row number with 3.

SELECT
    (@cnt := @cnt + 1) AS rowNumber,
    t.rowID
FROM myTable AS t
  CROSS JOIN (SELECT @cnt := 0) AS dummy
WHERE t.CategoryID = 1
ORDER BY t.rowID ;

Result

| ROWNUMBER | ROWID |
---------------------
|         1 |     1 |
|         2 |    25 |
|         3 |    33 |
|         4 |   150 |