Fetching subform for sales orders modules using COQL

I'm using the sales order module, I am trying to fetch a list of orders that belong to a customerId using COQL. While fetching the order, I am not able to retrieve order items(which is a subform). I am using v3 of coql.
I tried using the following thing:

Fetching Ordered_Items field in the query.
Query -

{
"select_query": "select Ordered_Items from Sales_Orders where id is not null"
}

Error:

{
"code": "INVALID_QUERY",
"details": {
"column_name": "Ordered_Items"
},
"message": "unsupported column",
"status": "error"
}

I also tried using joins following the link - https://www.zoho.com/crm/developer/docs/api/v3/COQL-Overview.html#joins

Query:

{
"select_query": "SELECT SO_Number, Subject, Grand_Total, Ordered_Items.Product_Name, Ordered_Items.Quantity, Ordered_Items.List_Price, Ordered_Items.Discount, Ordered_Items.Tax, Ordered_Items.Net_Total FROM Sales_Orders WHERE SO_Number = '6244979000000735014'"
}

Error:

{
"code": "INVALID_QUERY",
"details": {
"column_name": "Ordered_Items.Tax"
},
"message": "column given seems to be invalid",
"status": "error"
}

Can you help me fetch sales orders along with the ordered items?