Please help SQL noob with query!

[sambucaboy]

Hi all i have a problem with a simple query im hoping one of you clever sql junkies can help me

ok i have a product table and am listing product id with product cost like so:

SELECT x.id,x.cost
FROM product x

Now the problem occurs when i want to add another column which lists tha average cost of items. I have tried this:

SELECT x.id,x.cost,AVG(x.cost)
FROM product x
GROUP BY x.id,x.cost;

I have also tried union. But to no avail :( i presume i am using totally the wrong technique? please point me in the right direction! thx muchly