Referencing Aggregated Column Value in Where Clause

Question

Hello -
I'm trying to determine how I can accomplish the following in the most straightforward, efficient way.
Among other things, I'm selecting the following value from my table:
max(received_date) as last_received_dateI also need to evaluate the "last_received_date" value as a condition in my where clause. However, I can't reference my aliased "last_received_date" column value, and when I try to evaluate max(received_date) in the where clause, I get the "group function is not allowed here" error.
Does anyone know of a good workaround?
Thanks,
Christine

Answer 1

Hi,
Column aliases can be used in the ORDER BY clause: aside from that, they cannot be used in the same query where they are defined. The workarounds are
(a) define the alias in a sub-query, and use it in a super-query, like Someoneelse did, or
(b) repeat the aliased expression, as in the HAVING-clause, below.
Aggregate functions are computed after the WHERE-clause. (That explains why you can do things like
SELECT  MAX (received_date) last_received_date_2008
FROM    table_x
WHERE   TO_CHAR (received_date, 'YYYY')  = '2008';).
The HAVING-clause is like the WHERE-clause, but it is applied after the aggregate functions are computed, e.g.
SELECT    deptno
,         MAX (recieved_date)  AS last_received_date
FROM      table_x
GROUP BY  deptno
HAVING    MAX (received_date)    > SYSDATE - 7   -- Only show deptartments with activity in the last week
;