为什么两个表达式的结果不同？

Question

MySQL Expression One：

string userName = InputUser.Text.ToString().Trim();
string judgeStr = "select * from users where username='"+userName+"'";

MySQL Expression Two：

string userName = InputUser.Text.ToString().Trim();

string judgeStr = "select * from users where username='@username'";  
judgeCmd.Parameters.AddWithValue("@username", userName);

结果应该不一样？