Q249682: HOWTO: Change a Field’s Datatype using DAO

Article: Q249682
Product(s): Microsoft Visual Basic for Windows
Version(s): WINDOWS:5.0,6.0
Operating System(s): 
Keyword(s): kbDAOsearch kbJET kbGrpDSVBDB kbDSupport
Last Modified: 11-JAN-2001

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The information in this article applies to:

- Microsoft Visual Basic Professional Edition for Windows, versions 5.0, 6.0 
- Microsoft Visual Basic Enterprise Edition for Windows, versions 5.0, 6.0 
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SUMMARY
=======

Microsoft Access allows you to modify an existing field's data type. To do so
programmatically, Microsoft Jet 4.0 introduces the ALTER TABLE ALTER COLUMN DDL
statement. However, there is no equivalent for Microsoft Jet 3.5.

This article demonstrates a method to alter a field's data type using DAO
objects.

MORE INFORMATION
================

Modifying a field's data type requires the following steps:

1. Rename the old field.

2. Add a new field.

3. Copying the data from the old field to the new field.

4. Delete the old field.

If the table has any indexes or relations, the relationships and indexes must be
dropped prior to performing the steps above, then re-established after
completion of the steps above.

Microsoft Access handles indexes but not relationships when changing data types.
The Jet 4.0 ALTER TABLE ALTER COLUMN DDL statement has similar limitations.

The sample code provided handles both indexes and relationships. It also contains
error handling to roll back the changes and report on any problems.

The main procedure is ChangeFieldType. It takes the following arguments:

- db - an open Database object where the table resides.

- TableName - the name of the table where the field resides.

- FieldName - the name of the field to be changed.

- NewType - the new data type for the field.

- NewAllowZeroLength - new value for the AllowZeroLength property.

- NewAllowNulls - used to set the Required property of the new field.

- NewAttributes - used to set the Attributes property of the new field.

Note: This procedure is for illustration purposes only. For example, the
procedure copies only basic field properties. In addition to these basic field
properties, other field properties might also have to be copied. These
additional field properties include ValidationRule, ValidationText,
DecimalPlaces, and others, depending on the field type. In addition, the
procedure does not copy user-defined properties.

The other procedures, RecordRelationInfo, RecordIndexInfo, IsField, and
MakeArray, are helper procedures used by the main function.

Sample Code
-----------

This sample changes the CustomerID field in the Customers table from a five
character field to an eight character field.

The sample uses the Nwind database that comes with Visual Basic.

1. In Visual Basic, create a new Standard EXE project.
  Form1 is created by default.

2. Add a command button to Form1. Command1 is created by default.

3. On the Project menu, select References.
  In the References dialog, select the Microsoft DAO Object Library.

4. On the Project menu, select Add Module to add a Code Module.
  Module1 is created by default.

5. Paste the following code into the General Declarations section of Module1's
  Code Window:

  Option Compare Text
  Option Explicit

  Const CFT_Failed As Long = 55555

  Private Const R_NAME = 0, R_ATTRIBUTES = 1, R_TABLE = 2, R_FOREIGNTABLE = 3, R_FIELD = 4, R_FOREIGNFIELD = 5

  Private Const I_NAME = 0, I_PRIMARY = 1, I_UNIQUE = 2, I_REQUIRED = 3, I_IGNORENULLS = 4, I_CLUSTERED = 5, I_FIELD = 6, I_FIELDATTRIBUTES = 7

  Public Sub ChangeFieldType(db As Database, _
                            ByVal TableName As String, _
                            ByVal FieldName As String, _
                            ByVal NewType As Integer, _
                            Optional NewSize As Long, _
                            Optional NewAllowZeroLength As Boolean = False, _
                            Optional NewAllowNulls As Boolean = True, _
                            Optional NewAttributes As Long)

  ' User-defined properties are not maintained

    Dim td As TableDef, I As Index, R As Relation, F As Field

  ' loop iterators for Indexes, Fields, and Relations collections:
    Dim I1 As Long, F1 As Long, R1 As Long

    Dim colR As Collection, colI As Collection
    Dim E_Desc As String, Process As String, SubProcess As String, E As Error
    Dim TempFieldName As String, Suffix As Long, OldName As String
    Dim Temp As Variant
    Dim OrdinalPosition As Long

    Set colI = New Collection
    Set colR = New Collection
    On Error GoTo CFT_Err
    DBEngine(0).BeginTrans

  ' Enumerate relations and save/remove them

    DBEngine(0).BeginTrans
    Process = "Removing relations on [" & TableName & "]![" & FieldName & "]"
    SubProcess = ""
    For R1 = db.Relations.Count - 1 To 0 Step -1
      Set R = db.Relations(R1)
      If R.Table = TableName Then
        For F1 = 0 To R.Fields.Count - 1
          Set F = R.Fields(F1)
          If F.Name = FieldName Then
            RecordRelationInfo R, colR
            SubProcess = "Removing relation " & R.Name
            db.Relations.Delete R.Name
            Exit For
          End If
        Next F1
      ElseIf R.ForeignTable = TableName Then
        For F1 = 0 To R.Fields.Count - 1
          Set F = R.Fields(F1)
          If F.ForeignName = FieldName Then
            RecordRelationInfo R, colR
            SubProcess = "Removing relation " & R.Name
            db.Relations.Delete R.Name
            Exit For
          End If
        Next F1
      End If
    Next R1
    Set F = Nothing
    Set R = Nothing
    DBEngine(0).CommitTrans

  ' Enumerate indices and save/remove them

    DBEngine(0).BeginTrans
    Process = "Removing indexes on [" & TableName & "]![" & FieldName & "]"
    SubProcess = ""
    db.TableDefs.Refresh
    Set td = db(TableName)
    td.Indexes.Refresh
    For I1 = td.Indexes.Count - 1 To 0 Step -1
      Set I = td.Indexes(I1)
      If I.Foreign <> True Then
        For F1 = 0 To I.Fields.Count - 1
          Set F = I.Fields(F1)
          If F.Name = FieldName Then
            RecordIndexInfo I, colI
            SubProcess = "Removing index " & I.Name
            td.Indexes.Delete I.Name
            Exit For
          End If
        Next F1
      End If
    Next I1
    Set F = Nothing
    Set I = Nothing
    DBEngine(0).CommitTrans

  ' Rename Field

    DBEngine(0).BeginTrans
    Process = "Renaming field"
    SubProcess = ""
    td.Fields.Refresh
    Set F = td(FieldName)
    OrdinalPosition = F.OrdinalPosition   ' save this value

    ' determine a field name not in use
    Suffix = 0
    Do
      Suffix = Suffix + 1
      TempFieldName = "XXX" & Suffix
    Loop While IsField(td, TempFieldName)

    ' rename the field
    SubProcess = "to " & TempFieldName
    F.Name = TempFieldName

    Set F = Nothing
    DBEngine(0).CommitTrans

  ' Add new Field

    DBEngine(0).BeginTrans
    Process = "Adding new field"
    SubProcess = ""
    td.Fields.Refresh
    Set F = td.CreateField(FieldName, NewType)
    If NewSize Then F.Size = NewSize
    F.AllowZeroLength = NewAllowZeroLength
    F.Required = Not NewAllowNulls
    F.Attributes = NewAttributes
    F.OrdinalPosition = OrdinalPosition
    td.Fields.Append F
    Set F = Nothing
    Set td = Nothing
    DBEngine(0).CommitTrans

  ' Copy data

    DBEngine(0).BeginTrans
    Process = "Copying data from " & TempFieldName & " to " & FieldName
    SubProcess = ""
    db.Execute "UPDATE [" & TableName & "] SET [" & FieldName & "]=[" & _
                TempFieldName & "]", dbFailOnError
    DBEngine(0).CommitTrans

  ' Delete temporary field

    DBEngine(0).BeginTrans
    Process = "Deleting temporary field " & TempFieldName
    SubProcess = ""
    Set td = db(TableName)
    td.Fields.Delete TempFieldName
    DBEngine(0).CommitTrans

  ' Add back Indices

    DBEngine(0).BeginTrans
    Process = "Adding indexes back into table"
    SubProcess = ""
    Set td = db(TableName)
    td.Fields.Refresh
    td.Indexes.Refresh
    OldName = ""
    Set I = Nothing
    For Each Temp In colI
      If Temp(I_NAME) <> OldName Then
        If Not (I Is Nothing) Then   ' handle first time through case
          SubProcess = "Adding index " & I.Name
          td.Indexes.Append I
        End If
        Set I = td.CreateIndex(Temp(I_NAME))
        I.Primary = Temp(I_PRIMARY)
        I.Unique = Temp(I_UNIQUE)
        I.Required = Temp(I_REQUIRED)
        I.IgnoreNulls = Temp(I_IGNORENULLS)
        I.Clustered = Temp(I_CLUSTERED)
      End If
      Set F = I.CreateField(Temp(I_FIELD))
      F.Attributes = Temp(I_FIELDATTRIBUTES)  ' to handle descending index
      I.Fields.Append F
    Next Temp
    If Not (I Is Nothing) Then   ' handle case of no indexes
      SubProcess = "Adding index " & I.Name
      td.Indexes.Append I
    End If
    Set F = Nothing
    Set I = Nothing
    Set td = Nothing
    DBEngine(0).CommitTrans

  ' Add back relations

    DBEngine(0).BeginTrans
    Process = "Adding relations back into database"
    SubProcess = ""
    OldName = ""
    db.Relations.Refresh
    Set R = Nothing
    For Each Temp In colR
      If Temp(I_NAME) <> OldName Then
        If Not (R Is Nothing) Then   ' handle first time through case
          SubProcess = "Adding relation " & R.Name
          db.Relations.Append R
        End If
        Set R = db.CreateRelation(Temp(R_NAME), Temp(R_TABLE), _
                                  Temp(R_FOREIGNTABLE), Temp(R_ATTRIBUTES))
      End If
      Set F = R.CreateField(Temp(R_FIELD))
      F.ForeignName = Temp(R_FOREIGNFIELD)
      R.Fields.Append F
    Next Temp
    If Not (R Is Nothing) Then   ' if there are no indexes...
      SubProcess = "Adding relation " & R.Name
      db.Relations.Append R
    End If
    Set F = Nothing
    Set R = Nothing
    DBEngine(0).CommitTrans

  ' Commit all pending chhanges

    DBEngine(0).CommitTrans
    Exit Sub
    
  CFT_Abort:
    On Error Resume Next
    Set F = Nothing
    Set td = Nothing
    DBEngine(0).Rollback
    DBEngine(0).Rollback
    Err.Clear
    On Error GoTo 0
    Err.Raise CFT_Failed, "ChangeFieldType", E_Desc
    Exit Sub
    
  CFT_Err:
    E_Desc = "Error " & Process
    If SubProcess <> "" Then E_Desc = E_Desc & vbCrLf & SubProcess
    If DBEngine.Errors.Count = 0 Then
      E_Desc = E_Desc & vbCrLf & "Error " & Err.Number & " " & _
               Err.Description
    Else
      For Each E In DBEngine.Errors
        E_Desc = E_Desc & vbCrLf & "Error " & E.Number & " (" & _
                 E.Source & ") " & E.Description
      Next E
    End If
    Debug.Print E_Desc
    Resume CFT_Abort
  End Sub

  Private Sub RecordRelationInfo(ByVal R As Relation, colR As Collection)

  ' Records information regarding the relationship and its fields
  ' in the colR collection.

    Dim F1 As Long, F As Field
    For F1 = 0 To R.Fields.Count - 1
      Set F = R.Fields(F1)
      colR.Add MakeArray(R.Name, R.Attributes, R.Table, R.ForeignTable, _
                          F.Name, F.ForeignName)
    Next F1
  End Sub

  Private Sub RecordIndexInfo(ByVal I As Index, colI As Collection)

  ' Records information about fields in the index and about the index itself
  ' into the colI collection.

    Dim F1 As Long, F As Field
    For F1 = 0 To I.Fields.Count - 1
      Set F = I.Fields(F1)
      colI.Add MakeArray(I.Name, I.Primary, I.Unique, I.Required, _
                         I.IgnoreNulls, I.Clustered, F.Name, F.Attributes)
    Next F1
  End Sub

  Private Function IsField(td As TableDef, ByVal FieldName As String) _
          As Boolean

  ' Returns TRUE if a field exists in the table with the same name as
  '    specified in FieldName.
  ' Returns FALSE otherwise.

     Dim F As Field
     Err.Clear
     On Error Resume Next
     Set F = td(FieldName)
     IsField = Err.Number = 0
     Err.Clear
  End Function
   
  Private Function MakeArray(ParamArray X() As Variant) As Variant
    
    ' Does the same thing as the Array() function in VB6
    
      MakeArray = X
    
  End Function

6. If necessary, change the CFT_Failed constant to use an error number that
  conforms to your company's standards.

7. Paste the following code into the General Declarations section of Form1's
  Code Window:

    Private Sub Command1_Click()

       Dim strDB As String
       strDB = "c:\Program Files\Microsoft Visual Studio\VB98\Nwind.mdb"

       Dim db As DAO.Database
       Set db = DBEngine(0).OpenDatabase(strDB)
       ChangeFieldType db, "Customers", "CustomerID", dbText, 8
       db.Close
   
   End Sub

8. If necessary, modify strDB to use your Nwind database.

9. Run the sample project.
  Click the command button.
  End the project.

10. Examine the table in Microsoft Access or the Visual Basic Visual Database
  Manager add-in.
  Note that the field has been resized.

(c) Microsoft Corporation 1999, All Rights Reserved.
Contributions by Malcolm Stewart, Microsoft Corporation


REFERENCES
==========

For additional information, please see the following article in the Microsoft
Knowledge Base:

  Q217011 HOWTO:Copy a DAO Tabledef Including User-Defined Properties

Additional query words: kbdsupport kbgrpvbdb

======================================================================
Keywords          : kbDAOsearch kbJET kbGrpDSVBDB kbDSupport 
Technology        : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVBA500 kbVBA600 kbVB500 kbVB600
Version           : WINDOWS:5.0,6.0
Issue type        : kbhowto

=============================================================================

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