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Hi could you explain why you need to calculate min(age)?
I thought the question is for each age and power combination, what is are the lowest prices for those non-evil wands.
When you group the results by a column, you have to explicitly tell how to aggregate all the other columns you are selecting. Even though the ages are all the same inside a group, you have to use min(age) or max(age) or any other aggregation function.