Agregacje i operacje grupowania

Zapytania do bazy danych PostgreSQL w Javie

Miller Trujillo

Staff Software Engineer

Agregacje: COUNT, SUM, AVG

Podsumowuje wiele wierszy w jeden wynik
COUNT: liczba wierszy
SUM: suma wartości liczbowych
AVG: średnia wartości liczbowych

SELECT COUNT(*) AS total_books,
       AVG(publication_year) AS avg_year
FROM books;

|total_books   |avg_year |
|--------------|---------|
|14            |1999.14  |

GROUP BY do podsumowań

Dzieli wiersze na grupy przed zastosowaniem agregacji

SELECT publication_year, COUNT(*) AS books_written
FROM books
GROUP BY publication_year;

| publication_year | books_written |
| ---------------- | ------------- |
| 2017             | 1             |
| 2003             | 1             |
| 1997             | 1             |
| 2018             | 3             |
| 2014             | 1             |
| 1960             | 1             |

GROUP BY z wieloma kolumnami

Grupowanie według wielu kolumn dla dokładniejszych podziałów

SELECT publication_year, c.name, COUNT(*) AS books_written
FROM books b
LEFT JOIN categories c on c.category_id = b.category_id 
GROUP BY publication_year, c.name;

| publication_year | name             | books_written |
| ---------------- | ---------------- | ------------- |
| 1951             | Fiction          | 1             |
| 1960             | Fiction          | 1             |
| 2018             | Computer Science | 3             |
| 1965             | Science Fiction  | 1             |
| 2010             | Fantasy          | 1             |
| 2003             | Computer Science | 1             |

Filtrowanie grup z HAVING

WHERE filtruje wiersze przed grupowaniem
HAVING filtruje grupy po agregacji

SELECT publication_year, COUNT(*) AS books_written
FROM books b
GROUP BY publication_year
HAVING COUNT(*) > 2;

| publication_year | books_written |
| ---------------- | ------------- |
| 2018             | 3             |

Agregacje i grupowanie w Javie

String query = """SELECT publication_year, COUNT(*) AS books_written FROM books b
    GROUP BY publication_year
    HAVING COUNT(*) > ?;""";
try (PreparedStatement pstmt = conn.prepareStatement(query)) {
    pstmt.setInt(1, 2); // filter years with more than 2 books

    try (ResultSet rs = pstmt.executeQuery()) {
        while (rs.next()) {
            int year = rs.getInt("publication_year");
            int count = rs.getInt("books_written");
}   }   }

| publication_year | books_written |
| ---------------- | ------------- |
| 2018             | 3             |

Czas na ćwiczenia!

Zapytania do bazy danych PostgreSQL w Javie