Preserving NULL values in a Double Variable

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 开发者_Python百科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.

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.

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

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.