ordering by multiple columns

Hi!

I'm trying to order a list in which the first parameter to order is a
specific field, and the second should vary depending on a specific
condition. More explicit, I would like to do something like:

- ORDER BY a.column1, [b.column2 if c.value != 19 || c.column2 if
c.value==19]

That is to say, use a column if a value on each row isn't 19, OR use a
different column if it isn't. How could I solve this? I've got to have a
global order over my query, so I cannot separate it into two queries, etc
:-?

Thank you!