只需回到加权平均的定义即可，因此使用sum()s和除法：
select part, sum(rating * numreviews) / sum(numreviews) as rating_average
from a
group by part
order by part;
如果愿意，可以将其转换为小数：
select part,
cast(sum(rating * numreviews) / sum(numreviews) as decimal(8, 2)) as rating_average
from a
group by part
order by part;
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