In the last case you are ordering by the "numbers" 1 and 2, 2 and 1, or 2 and 1. These numbers are not aliases to columns, just values. For example
ORDER BY CASE...THEN X1 * X1 / 2 END
X1 * X1 / 2 gets evaluated and treated as a value. The value does not become an alias to a column.
To use the 1, 2, etc for columns
ORDER BY 1, 2
or
ORDER BY 2, 1
To use the above in your case, a logical IF can be use to write a query for each sort order required. This would be better if you want the best performance. Another option, a last resort, is ad hoc using EXEC.
Randy
Helpdesk: Perhaps Im not the only one that does not know what you are doing. 😉