我已经安装了wordpress，我正在尝试连接数据库。我看到这个错误：

Cannot select database
The database server could be connected to (which means your username and password is okay) but the database could not be selected.

Are you sure it exists?
Does the user root have permission to use the database?
On some systems the name of your database is prefixed with your username, so it would be like username_. Could that be the problem?
If you do not know how to set up a database you should contact your host. If all else fails you may find help at the WordPress Support Forums.

我已检查数据库名称、用户名和数据库主机是否正确，但仍无法解决错误。我已尝试使用127.0.0.1，但仍不成功。