String sql = "INSERT INTO users (username,password,email) VALUES (?,?,?);";
PreparedStatement pstmt = (PreparedStatement) conn.prepareStatement(sql,Statement.RETURN_GENERATED_KEYS);//傳入參數：Statement.RETURN_GENERATED_KEYS
pstmt.setString(1, user.getUsername());
pstmt.setString(2, user.getPassword());
pstmt.setString(3, user.getEmail());
pstmt.executeUpdate();//執行sql                                                                             int autoInckey = -1;
ResultSet rs = pstmt.getGeneratedKeys(); //獲取結果  
if (rs.next()) {
　　autoIncKey = rs.getInt(1);//取得ID
} else {
　　// throw an exception from here
}

SELECT MAX(id) FROM table

SELECT LAST_INSERT_ID() 或者 SELECT @@INDENTITY