How to filter DropDownList based on UserName in ASP.Net

At first you need to have the proper database.

Take the example of Northwind Database.

Link:Install Microsoft Northwind and Pubs Sample databases in SQL Server Management Studio

Now If you want to see the Customers Orders By his CustomerId then you need to write the query this way.

This is a test Query.

DECLARE @CustomerId VARCHAR(30)
SET @CustomerId = 'ALFKI'
SELECT OrderId, OrderDate 
FROM Orders WHERE CustomerId = @CustomerId

You need to Bind the GridView this way

HTML

<asp:TextBox ID="txtCustomerId" runat="server" Width="140" />
<asp:GridView ID="GridView1" HeaderStyle-BackColor="#3AC0F2" HeaderStyle-ForeColor="White"
    runat="server" AutoGenerateColumns="false">
    <Columns>
        <asp:BoundField DataField="OrderId" HeaderText="OrderId" ItemStyle-Width="30" />
        <asp:BoundField DataField="OrderDate" HeaderText="OrderDate" ItemStyle-Width="150" />
    </Columns>
</asp:GridView>

Namespaces

using System.Data;
using System.Configuration;
using System.Data.SqlClient;

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        this.BindGrid();
    }
}
 
private void BindGrid()
{
    string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    using (SqlConnection con = new SqlConnection(constr))
    {
        using (SqlCommand cmd = new SqlCommand("SELECT OrderId, OrderDate FROM Orders WHERE CustomerId = @CustomerId"))
        {
            cmd.Parameters.AddWithValue("@CustomerId", this.txtCustomerId.Text.Trim());
            using (SqlDataAdapter sda = new SqlDataAdapter())
            {                
                cmd.Connection = con;
                sda.SelectCommand = cmd;
                using (DataTable dt = new DataTable())
                {
                    sda.Fill(dt);
                    GridView1.DataSource = dt;
                    GridView1.DataBind();
                }
            }
        }
    }
}