C# - Constructing Paramater Query SQL - LIKE % In .cs And In Grid View?

I am trying to implement admin login and operators(india,australia,america) login, now operator id starts with 'IND123' 'AUS123' and 'AM123'

when operator india logs in he can see the details of only members have id 'IND%', the same applies for an australian and american users

and when admin logs in he can see details of members with id 'IND%' or 'AUS%' or 'AM%'

i have a table which defines the role i.e admin or operator and their prefix(IND,AUS respectively)

In loginpage i created a session for Role and prefix

PREFIX = myReader["Prefix"].ToString();
Session["LoginRole"] = myReader["Role"].ToString();
Session["LoginPrefix"] = String.Concat(PREFIX + "%"); works fine

In main page(after login) i have to count the number of member so i wrote

string prefix = Session["LoginPrefix"].ToString();
string role = Session["LoginRole"].ToString();
if (role.Equals("ADMIN"))
StrMemberId = "select count(*) from MemberList";
else
StrMemberId = "select count(*) from MemberList where MemberID like '"+prefix+"'";

Problem: 1. i want to constructor parameter something like

StrMemberId = "select count(*) from MemberList where MemberID like '@prefix+'";
myCommd.Parameters.AddWithValue("@prefix", prefix);

Which is not working

2 Displaying the members in gridview i need to give condition (something like if (role.Equals("ADMIN")) show all members else show member depending on the operator prefix)the list of members in operator mode and admin mode. - where to put the condition in gridview how to apply these