Change TField.Size at runtime

Posted on 1998-10-06
Last Modified: 2010-04-06
I have a table with a string field which is 6 characters in length. I want to change it to ten characters.

I need to do this at run time, so please don't suggest using Database Desktop!

Question by:philipleighs
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LVL 27

Expert Comment

ID: 1341963
What Database you use?

execute a sqlstatement in a sqlquerycomponoent

the statemant maybe like this:

alter table tablename modify(fieldname newdatatype)

LVL 27

Expert Comment

ID: 1341964

the statement above is only available on Oracle-databases


Expert Comment

ID: 1341965
The problem is then that you totally reconfigure your database table... Do you want that?

Paradox table?

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Expert Comment

ID: 1341966
this is an example how to change the paradox table in source :

Accepted Solution

ZifNab earned 50 total points
ID: 1341967
Hi philipleighs,

use for this the BDE api DBiDoRestructure :

Example 3: Alter a field in a Paradox or dBASE table.

This example will alter an existing field in a Paradox or dBASE table. NOTE: You must fill in all options in the ChangeRec with 0 or '' if the option is not used in the restructure. FillChar can be used to do this:

  Fillchar(MyChangeRec, sizeof(MyChangeRec), 0);

This example uses the following input:

  ChangeField(Table1, Table1.FieldByName('FOO'), MyChangeRec)

ChangeRec is defined as follows:


  ChangeRec = packed record
    szName: DBINAME;
    iType: Word;
    iSubType: Word;
    iLength: Word;
    iPrecision: Byte;

The function is defined as follows:

procedure ChangeField(Table: TTable; Field: TField; Rec: ChangeRec);

  Props: CURProps;
  hDb: hDBIDb;
  TableDesc: CRTblDesc;
  pFields: pFLDDesc;
  pOp: pCROpType;
  B: Byte;
  // Initialize the pointers...
  pFields := nil;
  pOp := nil;
  // Make sure the table is open exclusively so we can get the db handle...
  if not Table.Active then
    raise EDatabaseError.Create('Table must be opened to restructure');
  if not Table.Exclusive then
    raise EDatabaseError.Create('Table must be opened exclusively' +

      'to restructure');
  Check(DbiSetProp(hDBIObj(Table.Handle), curxltMODE, Integer(xltNONE)));
  // Get the table properties to determine table type...
  Check(DbiGetCursorProps(Table.Handle, Props));
  // Make sure the table is either Paradox or dBASE...
  if (Props.szTableType <> szPARADOX) and (Props.szTableType <> szDBASE) then
    raise EDatabaseError.Create('Field altering can only occur on Paradox' +
      ' or dBASE tables');

  // Allocate memory for the field descriptor...
  pFields := AllocMem(Table.FieldCount * sizeof(FLDDesc));
  // Allocate memory for the operation descriptor...
  pOp := AllocMem(Table.FieldCount * sizeof(CROpType));
    // Set the pointer to the index in the operation descriptor to put
    // crMODIFY (This means a modification to the record is going to happen)...
    Inc(pOp, Field.Index);
    pOp^ := crMODIFY;
    Dec(pOp, Field.Index);
    // Fill the field descriptor with the existing field information...

    Check(DbiGetFieldDescs(Table.Handle, pFields));
    // Set the pointer to the index in the field descriptor to make the
    // midifications to the field
    Inc(pFields, Field.Index);
    // If the szName portion of the ChangeRec has something in it, change it...
    if (Length(Rec.szName) > 0) then
      pFields^.szName := Rec.szName;
    // If the iType portion of the ChangeRec has something in it, change it...
    if (Rec.iType > 0) then

      pFields^.iFldType := Rec.iType;
    // If the iSubType portion of the ChangeRec has something in it, change it...
    if (Rec.iSubType > 0) then
      pFields^.iSubType := Rec.iSubType;
    // If the iLength portion of the ChangeRec has something in it, change it...
    if (Rec.iLength > 0) then
      pFields^.iUnits1 := Rec.iLength;
    // If the iPrecision portion of the ChangeRec has something
    // in it, change it...
    if (Rec.iPrecision > 0) then

      pFields^.iUnits2 := Rec.iPrecision;
    Dec(pFields, Field.Index);
    for B := 1 to Table.FieldCount do begin
      pFields^.iFldNum := B;
      Inc(pFields, 1);
    Dec(pFields, Table.FieldCount);
    // Blank out the structure...
    FillChar(TableDesc, sizeof(TableDesc), #0);
    //  Get the database handle from the table's cursor handle...
    Check(DbiGetObjFromObj(hDBIObj(Table.Handle), objDATABASE, hDBIObj(hDb)));

    // Put the table name in the table descriptor...
    StrPCopy(TableDesc.szTblName, Table.TableName);
    // Put the table type in the table descriptor...
    StrPCopy(TableDesc.szTblType, Props.szTableType);
    // The following three lines are necessary when doing any field restructure
    // operations on a table...
    // Set the field count for the table
    TableDesc.iFldCount := Table.FieldCount;
    // Link the operation descriptor to the table descriptor...

    TableDesc.pecrFldOp := pOp;
    // Link the field descriptor to the table descriptor...
    TableDesc.pFldDesc := pFields;
    // Close the table so the restructure can complete...
    // Call DbiDoRestructure...
    Check(DbiDoRestructure(hDb, 1, @TableDesc, nil, nil, nil, False));
    if (pFields <> nil) then
    if (pOp <> nil) then



Author Comment

ID: 1341968
Thanks Zif,

This is exactly what I needed.


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