finding a maximum or minimum sum

I have a table that looks like so:
userid | amount
---------------
1 | $500
2 | $400
2 | $-100
2 | $10
3 | $10
3 | $10

I run a select sum(amount) from payments group by userid
userid | sum
--------------
1 | $500
2 | $310
3 | $20

I need to modify this query so it returns the minimum, maximum and
average sums. Is there any way I can do this?

-Michael
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From: "Josh Berkus" <josh(at)agliodbs(dot)com>
Subject: Re: problem with Pl/Pgsql function
To: "Matteo Centenaro" <gente_che_mixa(at)hotmail(dot)com>, pgsql-sql(at)postgresql(dot)org
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Matteo,

> The error riported is:
>
> Fail to add null value in not null attribute prefisso
>
> The line which the error refere is:
>
> id_lista := nextval('tab_estrazioni_id_seq');
> insert into tab_estrazioni values (id_lista, pre, tel);
>
> Has Anybody any suggest? Thanks!

While my Italian isn't really good enough to analyze your function, the
above error seems simple. tab_estrazioni probably has the field
prefisso, which is defined as NOT NULL with no default. In the above
code, you're trying to add a record to tab_estrazioni filling in only
the columns id_lista, pre, and tel. Without a default or a supplied
value, this inserts NULL into the prefisso column, which the database
rejects.

Thus the solution is to either: a) Change prefisso to NULL, b) give
prefisso a default, or c) supply a value for prefisso. Unfortunatley,
the first two require dropping and re-creating the table.

-Josh Berkus

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