SQL AVG in subselect - sql

Question

I want to get rates from a product (productId) that is in a specific category (fk_rate_category) and the overall average rates from all products in the category.
To be more specific: I need the data for a chart. One line in the chart represents the rate of a product(y-axis = rate ; x-axis = date_added) and the other line shows the average rates of all products.
I had the following approach:
SELECT id, productId, rate, fk_rate_category, date_added,
(SELECT AVG(rate) AS Expr1
FROM rates
WHERE (fk_rate_category = r.fk_rate_category)) AS avgRate
FROM rates AS r
WHERE (productId = #productId) AND (fk_rate_category = #fk_rate_category)
The problem is, that with my approach the avgRates value is the same in every record returned.
Any idea?

Answer 1

Firstly, always try to avoid using a correlated subquery when you can achieve the same result with a join, the optimiser will deal with it much better and you'll get the results faster.
I think the problem could be because you are not linking the dates of the data as well as FK_Rate_Cateogry, I'd imagine if this is for graphical purposes you would want the average by date:
SELECT r.ID,
r.ProductID,
r.Rate,
r.FK_Rate_Category,
r.Date_Added,
ar.avgRate
FROM Rates r
LEFT JOIN
( SELECT r.Date_Added,
r.FK_Rate_Category,
AVG(r.Rate) AS avgRate
FROM Rates r
GROUP BY r.FK_Rate_Category, r.Date_Added
) ar
ON ar.FK_Rate_Category = r.FK_Rate_Category
AND ar.Date_Added = r.Date_Added
WHERE r.ProductID = #ProductID
AND r.FK_Rate_Category = #FK_Rate_Category;

Answer 2

You got the set of records which have the specific id and rate_category. And avgRate is an average value from all records with the same rate_category - this set has the outer one as subset(see conditions of both queries). So all returned records have the same avgRate value.
So I see some holes in your logic line.