November 2016
Intermediate to advanced
144 pages
3h 36m
Polish
Content preview from Pierwsze kroki z SQL
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Rozdział 6. Klauzule GROUP BY i ORDER BY
Agregowanie danych (ang. aggregating data) (zwane też zwijaniem, podsumowywaniem lub grupowaniem danych) polega na wyświetleniu pewnego rodzaju łącznych wartości zawartych w określonym zestawie wierszy. Do najczęściej stosowanych działań, dzięki którym otrzymamy taki właśnie wynik, należy między innymi sumowanie wartości, wyliczanie wartości minimalnej, maksymalnej oraz średniej. Za pomocą języka SQL możesz dokonać grupowania otrzymanych w ten sposób danych według dowolnie wybranych kolumn, dzięki czemu w prosty sposób uzyskasz pożądane zestawienie.
Grupowanie wierszy
Na początek wykonajmy prostą agregację danych: policzmy wiersze w tabeli. Aby tego dokonać, otwórz okno edytora języka SQL i wpisz poniższą ...
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.