PostgreSQL - Aliases column and HAVING

SELECT  
CASE WHEN SUM(X.Count)*3600 is null THEN  '0'  
            ELSE  
            SUM(X.Count)*3600  
       END AS PJZ,  
       X.Mass  
FROM X  
WHERE X.Mass > 2000  
HAVING ((X.Mass / PJZ * 100) - 100) >= 10;

Getting: ERROR: Column »pjz« doesn't exists.

How can I do something like this?