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I have to plan a plan tree based on relational algebra.My question is that at some point I have a SORT ORDER BY the output of a join, so how do you represent group by in relational algebra.

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Check this wiki article and look under the sections aggregation: It mentions a way to group by. Otherwise there's also which shows some group by. Maybe that can guide you in the direction – Allan S. Hansen Dec 13 '13 at 10:52
Thanks very much you've answered my question – Phil_Charly Dec 13 '13 at 10:58

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