Sort by value outside the Mysql query

Question

I need to sort the result of my search by a value that is outside the query.

For example, in the query I have a field called answer and with this value I make a calculation. But the result of that calculation is not in table.

My query

$search = mysql_query('SELECT * FROM simuladosrespondidos AS sr 
            JOIN usuarios AS u ON u.idUsuarios = '.$idUsuario.' AND sr.idUsuarios = u.idUsuarios 
            JOIN questoes AS q ON q.idQuestao = sr.idQuestao WHERE sr.idSimulado = '.$idSimulado.'');

What I do is I take the value of a column called answers and do the calculation on top of it. How to order by that calculation?

Note: The calculation q do actually is the following: I get the value of the answer column. Each user has several answers. If it equals 'C' we are + 1 in a variable here. If it equals 'E' I decrease - 1 and if it equals E I do nothing and at the end of everything I have a value and it is for that value I want to order.