It sounds like you want to filter your gridview based on the year you select in your drop down list. In your SQL, you can add a where clause to your SQL based on DateFrom, DateTo, or both, like:
Where DatePart(YEAR, PurchaseOrder.DateFrom) = @Year
then add a parameter to your query:
Dim cmd As ...
Dim intYear as Integer = DDYear.SelectedValue 'Get the year
cmd.Parameters.Add("@Year", SqlDbType.Int) 'Make a Parameter
cmd.Parameters("@Year").Value = intYear 'Fill parameter value
One problem in your code is that you are re-binding the DDYear to your new list, but DDYear is just the drop down list of years. You might want to do something like:
Remove this:
DDYear.DataSource = ds
DDYear.DataValueField = "2019,2020,2021"
DDYear.DataBind()
And add this:
Dim gvw As GridView = myGridView 'use your gridview name
gvw.DataSource = ds
gvw.DataBind()
This will be efficient, returning only the rows for the selected year, plus it loads the gridview instead of the drop down list with the new rows. Add your code to the OnSelectedIndexChanged event of DDYear. This will trigger it to refresh the gridview each time you change the year in your drop down list.
If you need a rows where the FiscalYear is the date used, then use that field instead of DateFrom. If you need purchase orders with a start or end date within the year, then add OR to the Where clause above using DatePart in the same way.