How do I retrieve data for an observable colleciton inside an object from SQL Server for WPF C#?

Question

I have a DB with multiple tables. for example :

• Customer : CustID, FirstName, LastName
• Order : OrderID, CustID, OrderName, Quantity

I'm using SqlCommand and SqlDataReader in Visual Studio (WPF C#) to retrieve the data.

My Customer can have multiple order. If I use a simple SELECT like :

SELECT * FROM Customer c JOIN Order o ON c.CustID = o.CustID

I obviously get multiple line depending on how many orders the customer have.

In C#, I retrieve my customer data

    private Customer GetCustomer(SqlDataReader dataReader){
    return = new Customer(int CustId: dataReader.GetInt32(0), 
    string FirstName: dataReader.GetString(1), 
    string LastName: dataReader.String(2), OrderCollection Order: ???)
    }

but I don't understand how to get each order inside the collection. Each line only has the information of one order, the next line has the customer information again. And I can't seem to figure out how to combine multiple.

I can only do one SQL query at the time or it starts over from the first line. I have multiple customers who have each multiple orders.

public override CustomersCollection GetCustomerDatas()
        {
            string sql = "SELECT * FROM Customer c JOIN Order o ON c.CustID = o.CustID";

            SqlCommand cmd = new SqlCommand(sql, SqlConnection);
            SqlDataReader dataReader = cmd.ExecuteReader();
            CustomersCollection customers= new CustomersCollection();
            while (dataReader.Read()){
                Customer c = GetCustomer(dataReader);
                if (c != null)
                {
                    customers.Add(c);
                }
            }
            dataReader.Close();
            return customers;
        }

Thanks in advance.