A "group by" is used to group selected rows in a table to perform arg functions on them. Rows not included in the "group by" clouse and not part of ARG functions cannot be used ...(logical grouping of data) or orerations performed (on groups of data) can be used with the "HAVING" clause to further refine the output. ex of some ARG functions : SUM() , MIN(),MAX(), AVG()

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