XML to SQL Server table

You're not making it very easy for us to help you by obviously not mentioning a lot of crucial details about your task!

I tried to do something - at least to insert the header information into the table (or updating it, if it already exists).

Take this is a learning guide - and extend it to also cover the SALES_DETAILS table, which I REALLY do not know how to handle since you're not telling us some many things about your setup....... learn from my sample and do the second step yourself, as a learning experience!

-- Load XML from file
DECLARE @XmlContent XML 

SELECT @XmlContent = CAST(BulkColumn AS XML)
FROM OPENROWSET(BULK 'H:\SDATA.xml', SINGLE_BLOB) AS e;

SELECT @XmlContent

-- declare a temp table (or table variable) to hold the data
DECLARE @TempTable TABLE (BillNo NUMERIC(9, 0), SalesDate DATE, CustomerID INT, DetailsXML XML)

-- insert header data and the details XML into temporary table
INSERT INTO @TempTable (BillNo, SalesDate, CustomerID, DetailsXML)
    SELECT
        BillNo = XCHDR.value('(BILLNO)[1]', 'numeric(9,0)'),
        SalesDate = XCHDR.value('(DATE)[1]', 'date'),
        CustomerID = XCHDR.value('(CUSTOMERCODE)[1]', 'int'),
        DetailsXml = XCHDR.query('SALESDETAILS')
    FROM
        @XmlContent.nodes('/SALESDATA/SALESHEADER') XTHDR(XCHDR)

SELECT * FROM @TempTable

-- update those rows that already exist in the SALES_HEADER table
UPDATE 
    INV_NO = tt.BillNo,
    [Date] = tt.SalesDate,
    [CUST_ID] = tt.CustomerID
FROM dbo.SALES_HEADER hdr
INNER JOIN @TempTable tt ON tt.BillNo = hdr.INV_NO

-- insert those rows that don't exist yet - record the newly inserted ID values
INSERT INTO dbo.SALES_HEADER (INV_NO, [Date], CUST_ID)
    SELECT tt.BillNo, tt.SalesDate, tt.CustomerID 
    FROM @TempTable tt
    WHERE NOT EXISTS (SELECT * FROM dbo.SALES_HEADER hdr WHERE hdr.INV_NO = tt.BillNo)