New field based on sum field I'm trying to create a new field based in another (sum field)

SELECT sum(price) AS old_price,
   old_price+100 AS total_price
FROM sales


This returns Unknown column 'old_price' in 'field list'

sum is just a example, my real issue is to use with a select inside this field

What am I doing wrong?

Thanks.

--edit
I've used variables to reuse the alias, updated code:

SELECT @old_price := sum(price) AS old_price,
   @old_price+100 AS total_price
FROM sales


If you think that it is not a good practice, tell me.
Yes, columns in the select-list cannot reference other columns by alias. This is standard SQL.

MySQL does not guarantee the order of evaluation of the select-list from left to right, so it would not be a recommended practice to use variables like you show.

Another workaround is to do computations and create column aliases in a derived table, and then reference them in the outer query. This is standard SQL and is reliable. 

SELECT old_price, old_price + 100 AS total_price
FROM (
    SELECT sum(price) AS old_price FROM sales
) AS t;