SELECT * FROM [DB_Table] WHERE ([Site] = @Site) AND (RoleType=@RoleType)

SELECT * FROM [DB_Table] WHERE ([Site] = @Site) AND (RoleType=@RoleType OR @RoleType <> NULL)

<asp:ListItem Text="All" Value="All" Selected="True"></asp:ListItem>
<asp:ListItem Text="Staff" Value="Staff"></asp:ListItem>
<asp:ListItem Text="Manager" Value="Manager"></asp:ListItem>
<asp:ListItem Text="Distributor" Value="Distributor"></asp:ListItem>

SELECT * FROM [DB_Table] WHERE ([Site] = @Site) AND (RoleType=@RoleType OR @RoleType = 'ALL')

SELECT * FROM [DB_Table] WHERE ([Site] = @Site) AND (RoleType=@RoleType OR @RoleType is NULL)