Executing Basic SQL Queries

Querying a PostgreSQL Database in Java

Miller Trujillo

Staff Software Engineer

Statements in JDBC

Use executeQuery() for SELECT statements
Use executeUpdate() for INSERT, UPDATE, DELETE

Statement as a messenger

Creating a 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)) {

Reading results with ResultSet

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)
...

Handling Exceptions

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

Recap

Statement sends SQL commands to the database
ResultSet holds query results, loop with next()
Getter methods: getInt(), getString(), getDate()
executeQuery() for SELECT, executeUpdate() for modifications
Handle errors with SQLException

Recap

Let's practice!

Querying a PostgreSQL Database in Java