Thanks Allan I've tried your method earlier, but only measure have aggregation option, date type variable doesn't. Another method I'd try is use "TreatAs" operator to create new variable from Infection_date. This can correctly change date variable into measure. But it comes up another two problem: 1. I can't create max or min for each group. Aggregation work only when we use object with group role, but object "Table" doesn't have this concept. 2. Even if we create min and max successfully. Since these two variables are measure, we can't give them any date-like format. then I stuck.....
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