How to return Data from multiple tables in DAL layer with one-to-many relationship using ADO.NET

While you can do a join, like you said you will get back multiple records for the each order duplicating customer information, which you code will have to de-duplicate.

I like to use SqlDataReader's support for multiple result sets to make this easier. https://msdn.microsoft.com/en-us/library/hh223698(v=vs.110).aspx

So you SQL will look something like this:

--Assume that @CustomerId is a parameter (always use parameters!)
SELECT CustomerId, CustomerName
  FROM Customer
 WHERE CustomerId = @CustomerId

SELECT OrderId, OrderDate --all your order columns
  FROM Orders
 WHERE CustomerId = @CustomerId

Then in your ADO.Net code:

using (var reader = command.ExecuteDataReader())
{
    while (reader.Read())
    {
        //read first result set
    }

    reader.NextResult();

    while (reader.Read())
    {
        //read secondresult set
    }
}

Or even better, use the async versions of ExecuteDataReader, Read, NextResult :-)

If you are using SQL 2016 this can be done very nicely with JSON path query to return a hierarchical structure of JSON which can then be deserialized into your objects, or possibly passed through to the client.

SELECT
            O.ORDER_ID orderId,
            O.ORDER_NAME orderName,
            O.ACTIVE_START orderDate,
            (SELECT
                OI.ORDER_ITEM_ID itemId,
                OI.NAME name,
                OI.COMMENTS comments
            FROM
                dbo.ORDER_ITEM OI
            WHERE
                O.ORDER_ID = OI.ORDER_ID
            FOR JSON PATH) items
        FROM
            dbo.[ORDER] O
        WHERE
            O.PERSON_ID = @PERSON_ID
        FOR JSON PATH;

Which would return something like

[
  {
    "orderId": 21,
    "orderName": "my first order",
    "orderDate": "2016-09-18T11:01:41",
    "items": [
      {
        "itemId": 41,
        "name": "pizza",
        "comments": "Extra Cheese Please"
      },
      {
        "itemId": 42,
        "name": "italian sandwich",
        "comments": "No peppers"
      }
    ]
  }
]...

There is a git project that has wrappers around ado for facilitating these kinds of calls at https://github.com/ahhsoftware/EzAdoVSSolution which might be something you'd be interested in.