using column alias to make operations

Hello,

Given this SQL:

SELECT c.name AS Customer, sum(rc.days_total) AS Days,
count(distinct(rc.id)) AS Cages FROM customers c INNER JOIN
reserved_cages rc ON c.ID=rc.customer_id GROUP BY c.name

Now I want to display the Days/Cages, and I want to use the column
aliases for it, like this:

SELECT c.name AS Customer, sum(rc.days_total) AS Days,
count(distinct(rc.id)) AS Cages, (Days/Cages) FROM customers c INNER
JOIN reserved_cages rc ON c.ID=rc.customer_id GROUP BY c.name

But this doesn't work, but this works:

SELECT c.name AS Customer, sum(rc.days_total) AS Days,
count(distinct(rc.id)) AS Cages, (sum(rc.days_total)/
count(distinct(rc.id))) AS AVG FROM customers c INNER JOIN
reserved_cages rc ON c.ID=rc.customer_id GROUP BY c.name

But I think this is double-time as It has to make twice the same
operations.

I thought I could use the column alias, because in the ORDER BY
clausule I can use them ...

thanks !

raimon