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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: en.wikipedia.org/wiki/Relational_algebra#Aggregation. It mentions a way to group by. Otherwise there's also cs.ulb.ac.be/public/_media/teaching/infoh417/sql2alg_eng.pdf 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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