C#查询MySQL数据库

select id 
from user 
where username=310501 
union all 
select id 
from wallet 
where username=310501

C#读取

 string str = "select id from user " +
                "where username=" + userInfo.UserName +
                " union all " +
                "select id from wallet " +
                "where username=" + userInfo.UserName;
       MySqlCommand sqlCommand = new MySqlCommand(str, client.MySqlConn);
            MySqlDataReader reader = sqlCommand.ExecuteReader();
            string nickname = "";
            int id = 0;
            while (reader.Read())
            {
                //nickname = reader.GetString("nickname");
                id = reader.GetInt32("id");
                Console.WriteLine(/*"昵称=" + nickname + */"+id=" + id);
            }
            reader.Close();

个人理解 select… union…
用于将不同表中查询相同列的数据