How to use a (repeating) aggregate function value with other columns from the table I use the aggregate function on - postgresql

Question

Problem: I have to count the number of times a certain user has a certificate and then return the users name, his number of certificates and the difference between the maximum number of certificates across all users and this specific users number of certificates. I succeeded in the first part (getting the number of certificates) which I'll denote as $query$ (because I have a feeling my problem has something to do with aliasing).
So $query$ looks like this:
User |N_Certificates
Geoff 4
Ann 2
Lisa 0
And my end result should look like this:
User |N_Certificates |Difference
Geoff 4 0
Ann 2 2
Lisa 0 4
I tried this query:
SELECT Sub.name, Sub.N_Certificates,
MAX(Sub.N_Certificates)- Sub.Certificates AS Difference FROM ($_query_$) AS SUB
but it returned a error (because I was trying to use an aggregate function in combination with a column I was not grouping by) or a wrong result (notably, difference=0 for all columns).
I tried a contraption with INNER JOIN on another version of sub (same $query$ code with another alias) but it also didn't work (same reason). I could ofcourse hard code the max but I don't think that's a good solution. My about screen tells me I'm using version 1.18 of pg_Admin.

Answer 1

You can't do it in this way, SQL syntax doesn't allow this.
The easiest way is to use a subquery:
SELECT Sub.name, Sub.N_Certificates,
(SELECT MAX(Sub.N_Certificates) FROM ($_query_$))
-
Sub.Certificates AS Difference
FROM ($_query_$) AS SUB
You can also use a common table expression:
WITH some_alias AS(
SELECT * FROM ($_query_$)
)
SELECT name, N_Certificates.
(SELECT MAX(N_Certificates)FROM some_alias)
-
Certificates AS Difference
FROM some_alias
And you can use a windows function: http://www.postgresql.org/docs/9.1/static/tutorial-window.html
SELECT Sub.name, Sub.N_Certificates,
MAX(Sub.N_Certificates) OVER ()
-
Sub.Certificates AS Difference
FROM ($_query_$) AS SUB