执行基础 SQL 查询

在 Java 中查询 PostgreSQL 数据库

Miller Trujillo

Staff Software Engineer

JDBC 中的 Statement

对 SELECT 使用 executeQuery()
对 INSERT、UPDATE、DELETE 使用 executeUpdate()

把 Statement 当作信使

创建 Statement

String sql = "SELECT book_id, title, publication_year FROM books";

try (Connection conn = DriverManager.getConnection(url, user, password);
     Statement stmt = conn.createStatement();
     ResultSet rs = stmt.executeQuery(sql)) {

用 ResultSet 读取结果

String sql = "SELECT book_id, title, publication_year FROM books";
try (Connection conn = DriverManager.getConnection(url, user, password);
     Statement stmt = conn.createStatement();
     ResultSet rs = stmt.executeQuery(sql)) {

  while (rs.next()) {

    int id = rs.getInt("book_id");
    String title = rs.getString("title");
    int year = rs.getInt("publication_year");
    System.out.println(id + " | " + title + " (" + year + ")");}}

1 | Effective Java (2018)
2 | Clean Code (2008)
...

处理异常

try {
    Statement stmt = conn.createStatement();

    stmt.executeQuery("SELECT * FROM unknown_table");

} catch (SQLException e) {

    System.err.println("Error: " + e.getMessage());
    System.err.println("SQLState: " + e.getSQLState()); 
}

Error: ERROR: relation "unknown_table" does not exist
  Position: 15
SQLState: 42P01

小结

Statement 向数据库发送 SQL 命令
ResultSet 保存结果，用 next() 循环
Getter 方法：getInt()、getString()、getDate()
查询用 executeQuery()，修改用 executeUpdate()
用 SQLException 处理错误

让我们来练习！

在 Java 中查询 PostgreSQL 数据库