field must appear in the GROUP BY clause or be used in an aggregate function?

Hey all.

I've hit an SQL problem that I'm a bit mystified by. I have two different
questions regarding this problem: why? and how do I work around it?

The following query:

SELECT GCP.id,
GCP.Name
FROM Gov_Capital_Project GCP,
WHERE TLM.TLI_ID = $2
group by GCP.id
ORDER BY gcp.name;

Produces the following error:

ERROR: column "gcp.name" must appear in the GROUP BY clause or be used in an aggregate function

That field is a CHAR, so I'm not sure what kind of aggregate to use,
or (more important to my understanding) why one is necessary.

As I said, I'm not sure I understand why this occurs. I'm assuming that I
don't understand "group by" as well as I thought I did ;)

This isn't my query, I'm translating a system prototyped in MSSQL to
Postgres. This query _does_ work in MSSQL. Does that constitute a
bug in MSSQL, or a shortcomming of Postgres, or just a difference of
interpretation?

--
Bill Moran
Potential Technologies
http://www.potentialtech.com