Hey @Stefano Bonicatti, I've found out I must attach the table previously to perform the SQL command. Something like this:

auto st = osquery::Registry::call("sql", "sql",
                                    {{"action", "attach"}, 
                                    {"table", "system_info"}}, response);

And then I can use the SQL engine with that table as usual

osquery::SQL sql("SELECT * FROM system_info;");

So, it seems like not all tables are loaded by default into extension registry