java - 如何将表名传递给 SELECT COUNT 查询中的准备语句？

Question

这个问题已经有答案了:

Is it possible to supply parameters for table or column name in Prepared Statements or QueryRunner.update()? (1 个回答)

Using Prepared Statements to set Table Name (8 个回答)

How to use a tablename variable for a java prepared statement insert [duplicate] (4 个回答)

已关闭 6 年前。

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)

我做错了什么？

Answer 1

您只能绑定(bind)PreparedStatement 中的值，而不能绑定(bind)语法元素或对象名称(在本例中为表名称)。您必须诉诸字符串操作:

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是值得怀疑的。