如何在 SELECT COUNT 查询中将表名传递给准备好的语句？ [复制]

Question

所以我有以下方法，效果很好：

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();
}

但是当我尝试时：

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();
}

我明白了：

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)

我做错了什么？

Answer 1

您只能绑定PreparedStatement 中的值，不能绑定语法元素或对象名称（在本例中为表名称）。您将不得不求助于字符串操作：

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

请注意，此查询中没有占位符，因此与普通的旧 Statement 相比，使用 PreparedStatement 是否真的有任何优势值得怀疑。