如何在SELECT COUNT查询中将表名传递给Prepared Statement? [英] How to pass table name to a Prepared Statement in a SELECT COUNT query?

问题描述

所以我有以下方法可以正常工作:

So I have this following method which works just fine:

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)

我在做什么错了?

推荐答案

您只能绑定PreparedStatement中的值，不能绑定语法元素或对象名(在这种情况下为表名).您将不得不求助于字符串操作:

You can only bind values in a PreparedStatement, not syntactic elements or object names (in this case, the table name). You'll have to resort to string manipulation:

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，因此使用PreparedStatement相对于普通的Statement是否真的有任何优势是令人怀疑的.

Note that there are no placeholders in this query, so it's questionable whether there's really any advantage in using a PreparedStatement as opposed to a plain old Statement.

这篇关于如何在SELECT COUNT查询中将表名传递给Prepared Statement?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！