Allowing null values in column definitions introduces three-valued logic into your application. A comparison can evaluate to one of three conditions:

• True

• False

• Unknown

Because null is considered to be unknown, two null values compared to each other are not considered to be equal. In expressions using arithmetic operators, if any of the operands is null, the result is null as well.

Comparison between any System.Data.SqlTypes will return a SqlBoolean. The IsNull function for each SqlType returns a SqlBoolean and can be used to check for null values. The following truth tables show how the AND, OR, and NOT operators function in the presence of a null value. (T=true, F=false, and U=unknown, or null.)

Understanding the ANSI_NULLS Option

            colname > NULL

            colname > @MyVariable

SELECT CustomerID, AccountNumber, TerritoryID
FROM AdventureWorks.Sales.Customer
WHERE TerritoryID IN (1, 2, 3)
   OR TerritoryID IS NULL

Null values are special, and their storage and assignment semantics differ across different type systems and storage systems. A Dataset is designed to be used with different type and storage systems.

This section describes the null semantics for assigning null values to a DataColumn in a DataRow across the different type systems.

DBNull.Value

This assignment is valid for a DataColumn of any type. If the type implements INullable, DBNull.Value is coerced into the appropriate strongly typed Null value.

SqlType.Null

All System.Data.SqlTypes data types implement INullable. If the strongly typed null value can be converted into the column's data type using implicit cast operators, the assignment should go through. Otherwise an invalid cast exception is thrown.

null

If 'null' is a legal value for the given DataColumn data type, it is coerced into the appropriate DbNull.Value or Null associated with the INullable type (SqlType.Null)

derivedUdt.Null

For UDT columns, nulls are always stored based on the type associated with the DataColumn. Consider the case of a UDT associated with a DataColumn that does not implement INullable while its sub-class does. In this case, if a strongly typed null value associated with the derived class is assigned, it is stored as an untyped DbNull.Value, because null storage is always consistent with the DataColumn's data type.

Note:
The Nullable<T> or Nullable structure is not currently supported in the DataSet.

Multiple Column (Row) Assignment

The default value for any System.Data.SqlTypes instance is null.

Nulls in System.Data.SqlTypes are type-specific and cannot be represented by a single value, such as DbNull. Use the IsNull property to check for nulls.

Null values can be assigned to a DataColumn as shown in the following code example. You can directly assign null values to SqlTypes variables without triggering an exception.

Example

Private Sub WorkWithSqlNulls()
    Dim table As New DataTable()

    ' Specify the SqlType for each column.
    Dim idColumn As DataColumn = _
      table.Columns.Add("ID", GetType(SqlInt32))
    Dim descColumn As DataColumn = _
      table.Columns.Add("Description", GetType(SqlString))

    ' Add some data.
    Dim row As DataRow = table.NewRow()
    row("ID") = 123
    row("Description") = "Side Mirror"
    table.Rows.Add(row)

    ' Add null values.
    row = table.NewRow()
    row("ID") = SqlInt32.Null
    row("Description") = SqlString.Null
    table.Rows.Add(row)

    ' Initialize variables to use when
    ' extracting the data.
    Dim isColumnNull As SqlBoolean = False
    Dim idValue As SqlInt32 = SqlInt32.Zero
    Dim descriptionValue As SqlString = SqlString.Null

    ' Iterate through the DataTable and display the values.
    For Each row In table.Rows
        ' Assign values to variables. Note that you 
        ' do not have to test for null values.
        idValue = CType(row("ID"), SqlInt32)
        descriptionValue = CType(row("Description"), SqlString)

        ' Test for null value with ID column
        isColumnNull = idValue.IsNull

        ' Display variable values in console window.
        Console.Write("isColumnNull={0}, ID={1}, Description={2}", _
          isColumnNull, idValue, descriptionValue)
        Console.WriteLine()
    Next row
End Sub

static private void WorkWithSqlNulls()
{
    DataTable table = new DataTable();

    // Specify the SqlType for each column.
    DataColumn idColumn =
        table.Columns.Add("ID", typeof(SqlInt32));
    DataColumn descColumn =
        table.Columns.Add("Description", typeof(SqlString));

    // Add some data.
    DataRow nRow = table.NewRow();
    nRow["ID"] = 123;
    nRow["Description"] = "Side Mirror";
    table.Rows.Add(nRow);

    // Add null values.
    nRow = table.NewRow();
    nRow["ID"] = SqlInt32.Null;
    nRow["Description"] = SqlString.Null;
    table.Rows.Add(nRow);

    // Initialize variables to use when
    // extracting the data.
    SqlBoolean isColumnNull = false;
    SqlInt32 idValue = SqlInt32.Zero;
    SqlString descriptionValue = SqlString.Null;

    // Iterate through the DataTable and display the values.
    foreach (DataRow row in table.Rows)
    {
        // Assign values to variables. Note that you 
        // do not have to test for null values.
        idValue = (SqlInt32)row["ID"];
        descriptionValue = (SqlString)row["Description"];

        // Test for null value in ID column.
        isColumnNull = idValue.IsNull;

        // Display variable values in console window.
        Console.Write("isColumnNull={0}, ID={1}, Description={2}",
            isColumnNull, idValue, descriptionValue);
        Console.WriteLine();
    }

isColumnNull=False, ID=123, Description=Side Mirror
isColumnNull=True, ID=Null, Description=Null

When comparing null values, it is important to understand the difference between the way the Equals method evaluates null values in System.Data.SqlTypes as compared with the way it works with CLR types. All of the System.Data.SqlTypes Equals methods use database semantics for evaluating null values: if either or both of the values is null, the comparison yields null. On the other hand, using the CLR Equals method on two System.Data.SqlTypes will yield true if both are null. This reflects the difference between using an instance method such as the CLR String.Equals method, and using the static/shared method, SqlString.Equals.

The following example demonstrates the difference in results between the SqlString.Equals method and the String.Equals method when each is passed a pair of null values and then a pair of empty strings.

Private Sub CompareNulls()
    ' Create two new null strings.
    Dim a As New SqlString
    Dim b As New SqlString

    ' Compare nulls using static/shared SqlString.Equals.
    Console.WriteLine("SqlString.Equals shared/static method:")
    Console.WriteLine("  Two nulls={0}", SqlStringEquals(a, b))

    ' Compare nulls using instance method String.Equals.
    Console.WriteLine()
    Console.WriteLine("String.Equals instance method:")
    Console.WriteLine("  Two nulls={0}", StringEquals(a, b))

    ' Make them empty strings.
    a = ""
    b = ""

    ' When comparing two empty strings (""), both the shared/static and
    ' the instance Equals methods evaluate to true.
    Console.WriteLine()
    Console.WriteLine("SqlString.Equals shared/static method:")
    Console.WriteLine("  Two empty strings={0}", SqlStringEquals(a, b))

    Console.WriteLine()
    Console.WriteLine("String.Equals instance method:")
    Console.WriteLine("  Two empty strings={0}", StringEquals(a, b))
End Sub

Private Function SqlStringEquals(ByVal string1 As SqlString, _
    ByVal string2 As SqlString) As String

    ' SqlString.Equals uses database semantics for evaluating nulls.
    Dim returnValue As String = SqlString.Equals(string1, string2).ToString()
    Return returnValue
End Function

Private Function StringEquals(ByVal string1 As SqlString, _
    ByVal string2 As SqlString) As String

    ' String.Equals uses CLR type semantics for evaluating nulls.
    Dim returnValue As String = string1.Equals(string2).ToString()
    Return returnValue
End Function

    private static void CompareNulls()
    {
        // Create two new null strings.
        SqlString a = new SqlString();
        SqlString b = new SqlString();

        // Compare nulls using static/shared SqlString.Equals.
        Console.WriteLine("SqlString.Equals shared/static method:");
        Console.WriteLine("  Two nulls={0}", SqlStringEquals(a, b));

        // Compare nulls using instance method String.Equals.
        Console.WriteLine();
        Console.WriteLine("String.Equals instance method:");
        Console.WriteLine("  Two nulls={0}", StringEquals(a, b));

        // Make them empty strings.
        a = "";
        b = "";

        // When comparing two empty strings (""), both the shared/static and
        // the instance Equals methods evaluate to true.
        Console.WriteLine();
        Console.WriteLine("SqlString.Equals shared/static method:");
        Console.WriteLine("  Two empty strings={0}", SqlStringEquals(a, b));

        Console.WriteLine();
        Console.WriteLine("String.Equals instance method:");
        Console.WriteLine("  Two empty strings={0}", StringEquals(a, b));
    }

    private static string SqlStringEquals(SqlString string1, SqlString string2)
    {
        // SqlString.Equals uses database semantics for evaluating nulls.
        string returnValue = SqlString.Equals(string1, string2).ToString();
        return returnValue;
    }

    private static string StringEquals(SqlString string1, SqlString string2)
    {
        // String.Equals uses CLR type semantics for evaluating nulls.
        string returnValue = string1.Equals(string2).ToString();
        return returnValue;
    }
}

The code produces the following output:

SqlString.Equals shared/static method:
  Two nulls=Null

String.Equals instance method:
  Two nulls=True

SqlString.Equals shared/static method:
  Two empty strings=True

String.Equals instance method:
  Two empty strings=True

Other Resources