I wasn't sure how to word the question and I know the wording is bad, but the question itself is pretty simple. 

This is what I'm trying to do.

```
 SELECT /*stuff*/
      , ISNULL (a.z, a.y) AS "c1"
      , SUM(a.x - c1) AS "c2"
 /* more stuff */

```

This is what happens.

**Invalid column name 'c1'.**

I'm not even sure if what I'm trying to do is possible. I can always do it elsewhere in the code, but I try to let the SQL do most of the work for me. 

I am using SQL Server 2008 R2.