Access read NULL or empty value

hi there,

i had this statement to open recordset


Rs.Open "SELECT Count(a.booksid) FROM orderdetails a, orders b WHERE a.rdate = '' AND a.ordersno = b.ordersno AND b.memid = '" & txtmemid.Text & "'", Cn, adOpenStatic, adLockReadOnly, adCmdText

but the problem is i need the SQL statement to check whether the

a.rdate

is equal to blank or NULL thats meant no value assigned, but it returns me with the value of 0, anyone nows why?

Rs.Open "SELECT Count(a.booksid) FROM orderdetails a, orders b WHERE a.rdate IS NULL '' AND a.ordersno = b.ordersno AND b.memid = '" & txtmemid.Text & "'", Cn, adOpenStatic, adLockReadOnly, adCmdText