Functional dependency: Difference between revisions

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* '''Composition''': If ''X'' → ''Y'' and ''Z'' → ''W'', then ''XZ'' → ''YW''<ref name="Singh2009">{{cite book|author=S. K. Singh|title=Database Systems: Concepts, Design & Applications|url=https://books.google.com/books?id=8PNCKe2SpRwC&pg=PA323|year=2009|orig-year=2006|publisher=Pearson Education India|isbn=978-81-7758-567-4|page=323}}</ref>
 
AnotherOne consequencecan ofalso derive the [[Armstrong's axiomaxioms#Additional isrules (Secondary Rules)|'''union''' and '''decomposition''']] rules from Armstrong's axioms:<ref name="SilberschatzKorth2010a"/><ref name="Garcia-MolinaUllman2009">{{cite book|author1=Hector Garcia-Molina|author2=Jeffrey D. Ullman|author3=Jennifer Widom|title=Database systems: the complete book|year=2009|publisher=Pearson Prentice Hall|isbn=978-0-13-187325-4|edition=2nd|page=73}} This is sometimes called the splitting/combining rule.</ref>
:''X'' → ''Y'' and ''X'' → ''Z'' [[if and only if]] ''X'' → ''YZ''