Getting SQL Server Merge Error - sql

Question

I am trying to update the product history's average rating number by using the merge function
MERGE ProductRankingHistory P
USING Review R ON P.PRHProduct = R.ProductID
WHEN MATCHED THEN
UPDATE
SET P.PRHAverageRating = (SELECT AVG(ReviewRateValue)
FROM Review
WHERE ProductID = P.PRHProduct
AND YEAR(ReviewRateDate) = PRHYear);
And I get this error:
Msg 8672, Level 16, State 1, Line 2
The MERGE statement attempted to UPDATE or DELETE the same row more than once. This happens when a target row matches more than one source row. A MERGE statement cannot UPDATE/DELETE the same row of the target table multiple times. Refine the ON clause to ensure a target row matches at most one source row, or use the GROUP BY clause to group the source rows.

Answer 1

Merge seems unnecessary for this logic. Something like:
UPDATE ProductRankingHistory P
SET PRHAverageRating = (SELECT AVG(r.ReviewRateValue)
FROM Review r
WHERE r.ProductID = P.PRHProduct AND YEAR(r.ReviewRateDate) = P.PRHYear
)
WHERE EXISTS (SELECT 1
FROM Review r
WHERE r.ProductID = P.PRHProduct AND YEAR(r.ReviewRateDate) = P.PRHYear
);
After writing this, I see that you are probably using SQL Server (or a similar database). The following might work:
UPDATE p
SET PRHAverageRating = rp.avg_ReviewRateValue
FROM ProductRankingHistory p JOIN
(SELECT r.ProductID, YEAR(r.ReviewRateDate) as yyyy, AVG(r.ReviewRateValue) as avg_ReviewRateValue
FROM Review r
GROUP BY r.ProductID, YEAR(r.ReviewRateDate)
) rp
ON rp.ProductId = p.PRHProduct AND rp.yyyy = p.prhYear;