On the expressive power of semijoin queries

    Clinical Orthopaedics and Related Research, Volume 91, Issue 2, 2004, Pages 93-98.

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    Keywords:
    semijoin operatorsemijoin queryrelational algebrasemijoindiscerning powerMore(6+)

    Abstract:

    The semijoin algebra is the variant of the relational algebra obtained by replacing the join operator by the semijoin operator. We provide an Ehrenfeucht-Fraïssé game, characterizing the discerning power of the semijoin algebra. This game gives a method for showing that queries are not expressible in the semijoin algebra.

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