Preserving NULL values in a Double Variable - vb.net

Question

I'm working on a vb.net application which imports from an Excel spreadsheet.
If rdr.HasRows Then
Do While rdr.Read()
If rdr.GetValue(0).Equals(System.DBNull.Value) Then
Return Nothing
Else
Return rdr.GetValue(0)
End If
Loop
Else
I was using string value to store the double values and when preparing the database statement I'd use this code:
If (LastDayAverage = Nothing) Then
command.Parameters.AddWithValue("#WF_LAST_DAY_TAG", System.DBNull.Value)
Else
command.Parameters.AddWithValue("#WF_LAST_DAY_TAG", Convert.ToDecimal(LastDayAverage))
End If
I now have some data with quite a few decimal places and the data was put into the string variable in scientific notation, so this seems to be the wrong approach. It didn't seem right using the string variable to begin with.
If I use a double or decimal type variable, the blank excel values come across as 0.0.
How can I preserve the blank values?
Note: I have tried
Variable as Nullabe(Of Double)
But when passing the value to the SQL insert I get: "Nullable object must have a value."
Solution:
Fixed by changing the datatype of the parameter in the sub I was calling and then using Variable.HasValue to do the conditional DBNull insert.

Answer 1

I don't know which API you're using to do database inserts, but with many of them, including ADO.NET, the proper way to insert nulls is to use DBNull.Value. So my recommendation is that you use Nullable(Of Double) in your VB code, but then when it comes time to do the insert, you'd substitute any null values with DBNull.Value.

Answer 2

You need the question mark ? to let Double (or any value type) can store null (or Nothing). E.g.:
Dim num as Double? = Nothing
Note the ? mark.
To store in the db:
If num Is Nothing Then
... System.DBNull.Value ...
Else
... num ...
End If
or better:
If num.HasValue Then
... System.DBNull.Value ...
Else
... num.Value ...
End If

Answer 3

I am posting an article HERE while I keep looking for how to acheive a solution to your situation, but the article might have another solution which is to remove the null values, and add a default value. If I find anything else I will post it.
When you set up a database (at least
in MS SQL Server) you can flag a field
as allowing NULL values and which
default values to take. If you look
through people's DB structures, you'll
see that a lot of people allow NULL
values in their database. This is a
very bad idea. I would recommend never
allowing NULL values unless the field
can logically have a NULL value (and
even this I find this only really
happens in DATE/TIME fields).
NULL values cause several problems. For starters, NULL values
are not the same as data values. A
NULL value is basically an undefined
values. On the ColdFusion end, this is
not terrible as NULL values come
across as empty strings (for the most
part). But in SQL, NULL and empty
string are very different and act very
differently. Take the following data
table for example:
id name
---------------
1 Ben
2 Jim
3 Simon
4 <NULL>
5 <NULL>
6 Ye
7
8
9 Dave
10
This table has some empty strings (id: 7, 8, 10) and some NULL values
(id: 4, 5). To see how these behave
differently, look at the following
query where we are trying to find the
number of fields that do not have
values:
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* SELECT
* (
* SELECT
* COUNT( * )
* FROM
* test t
* WHERE
* LEN( t.name ) = 0
* ) AS len_count,
* (
* SELECT
* COUNT( * )
* FROM
* test t
* WHERE
* t.name IS NULL
* ) AS null_count,
* (
* SELECT
* COUNT( * )
* FROM
* test t
* WHERE
* t.name NOT LIKE '_%'
* ) AS like_count,
* (
* SELECT
* COUNT( * )
* FROM
* test t
* WHERE
* t.name IS NULL
* OR
* t.name NOT LIKE '_%'
* ) AS combo_count
This returns the following record:
LEN Count: 3
NULL Count: 2
LIKE Count: 3
Combo Count: 5
We were looking for 5 as records 4, 5, 7, 8, and 10 do not have values
in them. However, you can see that
only one attempt returned 5. This is
because while a NULL value does NOT
have a length, it is not a data type
that makes sense with length. How can
nothing have or not have a length?
It's like asking "What does that math
equation smell like?" You can't make
comparisons like that.
So, allowing NULL values makes you work extra hard to get the kind of
data you are looking for. From a
related angle, allowing NULL values
reduces your convictions about the
data in your database. You can never
quite be sure if a value exists or
not. Does that make you feel safe and
comfortable when programming?
Furthermore, while running LEN() on a NULL value doesn't act as you
might think it to, it also does NOT
throw an error. This will make
debugging your code even harder if you
do not understand the difference
between NULL values and data values.
Bottom line: DO NOT ALLOW NULL VALUES unless absolutely necessary.
You will only be making things harder
for yourself.