如何將表名稱傳遞給SELECT COUNT查詢中的Prepared Statement？

Question

所以我有這個以下方法，非常有效：

static void getCount(final String url, final String username, final String password) throws SQLException { 
    final Connection connection = DriverManager.getConnection(url, username, password); 

    final String query = "SELECT COUNT(*) FROM app_user"; 
    final PreparedStatement preparedStatement = connection.prepareStatement(query); 
    final ResultSet resultSet = preparedStatement.executeQuery(); 

    resultSet.next(); 
    System.out.println(resultSet.getInt(1)); 

    resultSet.close(); 
    preparedStatement.close(); 
    connection.close(); 
} 

但是當我嘗試：

static void foobar(final String url, final String username, final String password, final String tablename) throws SQLException { 
    final Connection connection = DriverManager.getConnection(url, username, password); 

    final String query = "SELECT COUNT(*) FROM ? "; 
    final PreparedStatement preparedStatement = connection.prepareStatement(query); 
    preparedStatement.setString(1, tablename); 
    final ResultSet resultSet = preparedStatement.executeQuery(); 

    resultSet.next(); 
    System.out.println(resultSet.getInt(1)); 

    resultSet.close(); 
    preparedStatement.close(); 
    connection.close(); 
} 

我得到：

Exception in thread "main" com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ''app_user'' at line 1 
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) 
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:57) 
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) 
    at java.lang.reflect.Constructor.newInstance(Constructor.java:526) 

我在做什麼錯？

Answer 1

只能綁定PreparedStatement中的值，而不能綁定語法元素或對象名稱（在本例中爲表名）中的值。你必須訴諸字符串操作：

final String query = String.format("SELECT COUNT(*) FROM %s", tablename); 
final PreparedStatement preparedStatement = connection.prepareStatement(query); 
final ResultSet resultSet = preparedStatement.executeQuery(); 

注意，有在此查詢沒有佔位符，所以這是值得懷疑是否真的在使用PreparedStatement，而不是一個普通的老Statement任何優勢。