Arbeiten mit Joins und Unterabfragen

Abfragen einer PostgreSQL-Datenbank in Java

Miller Trujillo

Staff Software Engineer

Daten in mehreren Tabellen

books, authors, categories, book_reviews
Daten über Tabellen hinweg kombinieren

Beziehungen in der Bibliotheksdatenbank

Tabellen verbinden

String sql = "SELECT b.title, a.first_name, a.last_name " +
             "FROM books b " +

             "INNER JOIN book_authors ba ON b.book_id = ba.book_id " +
             "INNER JOIN authors a ON ba.author_id = a.author_id " +

             "WHERE b.publication_year > ?";

PreparedStatement pstmt = conn.prepareStatement(sql);
pstmt.setInt(1, 2010);
ResultSet rs = pstmt.executeQuery();

| title                                              | first_name | last_name |
| -------------------------------------------------- | ---------- | --------- |
| Effective Java                                     | Joshua     | Bloch     |
| Designing Data-Intensive Applications              | Martin     | Kleppmann |

INNER vs LEFT JOIN

-- INNER JOIN: nur Übereinstimmungen
SELECT b.title, r.rating
FROM books b
INNER JOIN book_reviews r 
    ON b.book_id = r.book_id;

-- LEFT JOIN: alle linken Zeilen
SELECT b.title, r.rating
FROM books b
LEFT JOIN book_reviews r 
    ON b.book_id = r.book_id;

Inner und Left Joins

Unterabfragen

// Das zuletzt veröffentlichte Buch extrahieren
String sql = "SELECT title FROM books " +
             "WHERE publication_year = ( " +

             "  SELECT MAX(publication_year) FROM books " +
             ")";

PreparedStatement pstmt = conn.prepareStatement(sql);
ResultSet rs = pstmt.executeQuery();

| title                                              |
| -------------------------------------------------- |
| Effective Java                                     |
| Refactoring: Improving the Design of Existing Code |

Unterabfragen mit Parametern

// Alle "Fantasy"-Bücher extrahieren
String sql = "SELECT title FROM books " +
             "WHERE category_id IN ( " +

             "  SELECT category_id FROM categories WHERE name = ? " +
             ")";

PreparedStatement pstmt = conn.prepareStatement(sql);
pstmt.setString(1, "Fantasy");
ResultSet rs = pstmt.executeQuery();

| title                                    |
| ---------------------------------------- |
| The Way of Kings                         |
| Harry Potter and the Philosopher's Stone |

StringBuilder

String-Verkettung mit + ist langsam und schwer lesbar
append() fügt Text hinzu, toString() gibt das Ergebnis zurück

StringBuilder sb = new StringBuilder("SELECT * FROM ");

sb.append("books");

sb.append(" WHERE 1=1");

sb.toString(); // SELECT * FROM books WHERE 1=1

Lass uns üben!

Abfragen einer PostgreSQL-Datenbank in Java