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I have a point shapefile layer with several thousand points. Each record has a timestamp field and another field which defines a category to which that record belongs. There are about 200 different categories. Is there a way to generate a query to select the first point of each category? A solution that sets a flag in a new calculated field would work, and then a query could be made based on that field.

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You can use select by expression with this expression - or you can use the expression as a condition in a larger expression:

time = array_min (array_agg ( time, group_by:=category))

time and category are the names of the corresponding attributes: replace these to match the names you have.

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