How to deal with null values in F# - f#

Question

I am trying to fetch data(type:double) from MS access below. There are number of null values stored in A&B below. is there a way to change those null values to zeros?
let query sql w=
seq{
let conn = new OleDbConnection( #"Provider=Microsoft.ACE.OLEDB.12.0;
Data Source=Portfolio.accdb;
Persist Security Info=False;" )
conn.Open()
let DAdapter = new OleDbDataAdapter(sql,conn)
let DTable = new DataSet()
let i= DAdapter.Fill(DTable)
let rowCol = DTable.Tables.[0].Rows
let rowCount = rowCol.Count
for i in 0 .. (rowCount - 1) do
yield w (rowCol.[i])
}
type Table1= {
A:double;
B:double}
let cf=query "SELECT * FROM T" (fun row ->
{
A=unbox(row.["A"]);
B=unbox(row.["B"]);})

Answer 1

Define a function
let toFloat = function
| null -> 0.0
| obj -> unbox obj
And then use it as follows
let cf = query "SELECT * FROM T" (fun row ->
{ A = toFloat row.["A"]
B = toFloat row.["B"] } )

Answer 2

Maybe, your columns in DB have different type (for example, int and double). Or try check return value with DBNull type:
let toDouble x =
if System.Convert.IsDBNull(x) then 0.0
else System.Double.Parse(x.ToString())
To check I create that table:
And with your code:
open System.Data
open System.Data.OleDb
let toDouble x =
if System.Convert.IsDBNull(x) then 0.0
else System.Double.Parse(x.ToString())
let query sql w=
seq{
let conn = new OleDbConnection( #"Provider=Microsoft.ACE.OLEDB.12.0;
Data Source=F:/Portfolio.accdb;Persist Security Info=False;" )
conn.Open()
let DAdapter = new OleDbDataAdapter(sql,conn)
let DTable = new DataSet()
let i = DAdapter.Fill(DTable)
let rowCol = DTable.Tables.[0].Rows
let rowCount = rowCol.Count
for i in 0 .. (rowCount - 1) do
yield w (rowCol.[i])
conn.Close()
}
type Table1= { A:double; B:double }
let cf = query "SELECT * FROM T" (fun row -> { A = toDouble row.["A"]; B = toDouble row.["B"] } )
cf |> Seq.iter(fun x -> printfn "%A" x)
Result:
{A = 1.0;
B = 2.2;}
{A = 3.0;
B = 0.0;}
{A = 4.0;
B = 0.0;}