Postgres: GROUP BY several column - sql

Question

I have two table in this example.
( example column name )
First is the product
product_id | product_text
Second table is Price.
price_productid | price_datestart | price_price
Let's just say I have multiple datestart with the same product. How can I get the actual price ?
If I use GROUP BY in Postgres, with all the selected column, 2 row may come for the same product. Because the column price_datestart is different.
Example :
product_id : 1
product_text : "Apple Iphone"
price_productid : 1
price_datestart :"2013-10-01"
price_price :"99"
price_productid : 1
price_datestart :"2013-12-01"
price_price :"75"
If I try this :
SELECT price_productid,price_datestart,price_price,product_text,product_id
WHERE price_datestart > now()
GROUP BY price_productid,price_datestart,price_price,product_text,product_id
ORDER BY price_datestart ASC
It will give me a result, but two rows and I need one.

Answer 1

Use distinct on syntax. If you want current price:
select distinct on (p.productid)
p.productid, pr.product_text, p.price, p.datestart
from Price as p
left outer join Product as pr on pr.productid = p.productid
where p.datestart <= now()
order by p.productid, p.datestart desc
sql fiddle demo

Answer 2

You have a few problems, but GROUP BY is not one of them.
First, although you have a datestart you don't have a dateend. I'd change datestart to be a daterange, for example:
CREATE TABLE product
(
product_id int
,product_text text
);
CREATE TABLE price
(
price_productid int
,price_daterange TSRANGE
,price_price NUMERIC(10,2)
);
The TSRANGE allows you to set up validity of your price over a given range, for example:
INSERT INTO product VALUES(1, 'phone');
INSERT INTO price VALUES(1, '[2013-08-01 00:00:00,2013-10-01 00:00:00)', 199);
INSERT INTO price VALUES(1, '[2013-10-01 00:00:00,2013-12-01 00:00:00)', 99);
INSERT INTO price VALUES(1, '[2013-12-01 00:00:00,)', 75);
And that makes your SELECT much more simple, for example:
SELECT price_productid,price_daterange,price_price,product_text,product_id
FROM product, price
WHERE price_daterange #> now()::timestamp
AND product_id = price_productid
This also has the benefit of allowing you to query for any arbitrary time by swapping out now() for another date.
You should read up on ranges in PostgresQL as they are very powerful. The example above is not complete in that it should also have indices on price_daterange to ensure that you do not have overlaps for any product.
SQL fiddle with above solution