PostgreSQL query with generated columns - sql

Question

I have a schema as show like the below, and I want to run a query where I get a column in the output for every row of the points table.
So for each usage row I want to multiply the amount of the usage times the amount for the referenced points_id, and then sum that up and group by person. So for the example data I'd want output that looked like this:
Name | foo | bar | baz
-------|------|------|------
Scott | 10.0 | 24.0 | 0.0
Sam | 0.0 | 0.0 | 46.2
Here's the schema/data:
CREATE TABLE points (
ident int primary key NOT NULL,
abbrev VARCHAR NOT NULL,
amount real NOT NULL
);
CREATE TABLE usage (
ident int primary key NOT NULL,
name VARCHAR NOT NULL,
points_id integer references points (ident),
amount real
);
INSERT INTO points (ident, abbrev, amount) VALUES
(1, 'foo', 1.0),
(2, 'bar', 2.0),
(3, 'baz', 3.0);
INSERT INTO usage (ident, name, points_id, amount) VALUES
(1, 'Scott', 1, 10),
(2, 'Scott', 2, 12),
(3, 'Sam', 3, 3.4),
(4, 'Sam', 3, 12);
I'm using PostgreSQL 9.2.8
The data is just sample. There are thousands of rows in the real usage table and probably a dozen in the points table. The real intent here is I don't want to hardcode all the points summations as I use them in many functions.

Answer 1

select
t1.name,
sum(case when t2.abbrev='foo' then t1.amount*t2.amount else 0 end) as foo,
sum(case when t2.abbrev='bar' then t1.amount*t2.amount else 0 end) as bar,
sum(case when t2.abbrev='baz' then t1.amount*t2.amount else 0 end) as baz
from usage t1 inner join points t2 on t1.points_id=t2.ident
group by t1.name;
SQL Fiddle Example:http://sqlfiddle.com/#!15/cc84a/6;
Use following PostgreSQL function for dynamic cases:
create or replace function sp_test()
returns void as
$$
declare cases character varying;
declare sql_statement text;
begin
select string_agg(concat('sum(case when t2.abbrev=','''',abbrev,'''',' then t1.amount*t2.amount else 0 end) as ', abbrev),',') into cases from points;
drop table if exists temp_data;
sql_statement=concat('create temporary table temp_data as select
t1.name,',cases ,'
from usage t1 inner join points t2 on t1.points_id=t2.ident
group by t1.name ');
execute sql_statement;
end;
$$
language 'plpgsql';
Function uses temporary table to store dynamic columns data.
Call function in following way to get data:
select * from sp_test(); select * from temp_data;