sql select query with column 'AS' assignment

Not sure if this is the right newsgroup to use. I did not see a general
one or sql statement one for postgres. Please point me to the correct
location if this is wrong and I apologize for the off topic if it is.

I am attempting to do a select in which I force an existing
column(field) to a specific value. I need to do so in order to group
data properly.

ie:
agi_timesheets=# select distinct
employee_id,first_name,last_name,date,sum(hours),"R" as job_code from
timesheet group by employee_id,first_name,last_name,date having job_code
<> 'H' and job_code <> 'V' and date >= '01-01-2002' and date <=
'01-15-2002';
ERROR: Attribute 'R' not found

agi_timesheets=# select distinct
employee_id,first_name,last_name,date,sum(hours),job_code AS "R" from
timesheet group by employee_id,first_name,last_name,date having job_code
<> 'H' and job_code <> 'V' and date >= '01-01-2002' and date <=
'01-15-2002';
ERROR: Attribute timesheet.job_code must be GROUPed or used in an
aggregate function

agi_timesheets=# select distinct
employee_id,first_name,last_name,date,sum(hours),job_code AS 'R' from
timesheet group by employee_id,first_name,last_name,date having job_code
<> 'H' and job_code <> 'V' and date >= '01-01-2002' and date <=
'01-15-2002';
ERROR: parser: parse error at or near "'"

agi_timesheets=# select distinct
employee_id,first_name,last_name,date,sum(hours),job_code = 'R' from
timesheet group by employee_id,first_name,last_name,date having job_code
<> 'H' and job_code <> 'V' and date >= '01-01-2002' and date <=
'01-15-2002';
ERROR: Attribute timesheet.job_code must be GROUPed or used in an
aggregate function

etc. etc. etc. I have tried all possible combinations (or I think so)
of "R", 'R', R using = or AS on either side of job_code. Nothing seems
to work.

Several of these combinations work in MySQL, Access, and Oracle. Or at
least according to those online I have spoke to they do.

Can any explain to me what I am doing wrong? If this is possible in
PostgreSQL? Or the proper way of doing this? Or even a source of
information that explains it. The closest source I found was obviously
the psql documentation but I have yet to find a specific example of what
I am doing.

Thanks.