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?
您只能绑定PreparedStatement
中的值,而不能绑定语法元素或对象名称(在本例中为表名(。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,因此使用PreparedStatement
而不是普通的旧Statement
是否真的有任何优势是值得怀疑的。