Before continue the second bug report, I want to share some information about TClientDataSet.

From A ClientDataSet in Every Database Application by Cary Jensen, ClientDataSet has 2 data stores. One is delta. It represents the change log that all changes made to Data. Specifically, for each record that was inserted or deleted from Data, there resides a corresponding record in Delta. For modified records it is slightly different. The change log contains two records for each record modified in Data. One of these is a duplicate of the record that was originally modified. The second contains the field-by-field changes made to the original record.

If inserts a record, the TClientDataSet.Delta data packet will be as:

<?xml version="1.0" standalone="yes" ?>
<DATAPACKET Version="2.0">
<METADATA>
    <FIELDS>
      <FIELD attrname="ID" fieldtype="string" WIDTH="10" />
      <FIELD attrname="NAME" fieldtype="string" WIDTH="10" />
      <FIELD attrname="AGE" fieldtype="i4" />
     </FIELDS>
    <PARAMS DATASET_DELTA="1" />
   </METADATA>
  <ROWDATA>
    <ROW RowState="1" ID="S001" NAME="PETER" AGE="10" />
    <ROW RowState="4" ID="S002" NAME="JOHN" AGE="20" />
   </ROWDATA>
</DATAPACKET>

If deletes the record, the TClientDataSet.Delta data packet will be as:

<?xml version="1.0" standalone="yes" ?>
<DATAPACKET Version="2.0">
<METADATA>
    <FIELDS>
      <FIELD attrname="ID" fieldtype="string" WIDTH="10" />
      <FIELD attrname="NAME" fieldtype="string" WIDTH="10" />
      <FIELD attrname="AGE" fieldtype="i4" />
     </FIELDS>
    <PARAMS DATASET_DELTA="1" />
   </METADATA>
  <ROWDATA>
    <ROW RowState="2" ID="S001" NAME="PETER" AGE="10" />
   </ROWDATA>
</DATAPACKET>

If updates the record by setting age = 20, the TClientDataSet.Delta data packet will be as:

<?xml version="1.0" standalone="yes" ?>
<DATAPACKET Version="2.0">
<METADATA>
    <FIELDS>
      <FIELD attrname="ID" fieldtype="string" WIDTH="10" />
      <FIELD attrname="NAME" fieldtype="string" WIDTH="10" />
      <FIELD attrname="AGE" fieldtype="i4" />
     </FIELDS>
    <PARAMS DATASET_DELTA="1" />
   </METADATA>
  <ROWDATA>
    <ROW RowState="1" ID="S001" NAME="PETER" AGE="10" />
    <ROW RowState="8" AGE="20" />
   </ROWDATA>
</DATAPACKET>

You may notice that a additional row of data in the data packet. The first row is the orginal contents of the record while the second one is the change(s) we made to the data. Passing the orginal contents of row is because:
1. primary key(s) may not be available in the second row
2. the record may be updated by others while we edit the record

In the second row of data packet, you can find only the changes made to data is included. RowState is an indicator telling MIDAS server what to do with the values in that row, like insert, delete or update. Well, It's time back to my second bug report. What happens if updates the record by setting AGE = Null? The delta data packet is like this:

<?xml version="1.0" standalone="yes" ?>
<DATAPACKET Version="2.0">
<METADATA>
    <FIELDS>
      <FIELD attrname="ID" fieldtype="string" WIDTH="10" />
      <FIELD attrname="NAME" fieldtype="string" WIDTH="10" />
      <FIELD attrname="AGE" fieldtype="i4" />
     </FIELDS>
    <PARAMS DATASET_DELTA="1" />
   </METADATA>
  <ROWDATA>
    <ROW RowState="1" ID="S001" NAME="PETER" AGE="10" />
    <ROW RowState="8" />
   </ROWDATA>
</DATAPACKET>

RowState tells us the record is modified but NO information about the change on AGE field is listed in second row. This is because XML does not support null attribute value. That's why I say ClientDataSet cannot update fields with null value. Two bug reports in Borland Quality Centre has been raised about this issue in 2002 and 2004 but they are still open :(
Report#617
Report#8137

I do not know how other developers avoid this issue in their application but I know the users are quite unhappy.