Using DSN-Less Connections

While it is possible to programmatically create the DSN (see, for example, HOWTO: Programmatically Create a DSN for SQL Server with VB (Q184608)), I find it's more flexible to use a DSN-less connection.

The code that follows shows one way to convert existing linked tables to use a DSN-less connection. In the example, I'm creating a DSN-less link for SQL Server, using Trusted Connection. If you're going against a different DBMS, or want different options connecting to SQL Server, check out the various ODBC DSN-Less connection strings that Carl Prothman lists at Connection Strings

Create your linked tables using a DSN, and then convert them this way before you deploy the application. When you create the linked tables, you may be asked to indicate the Unique Identifiers for the linked table. This happens when the object to which you're linking does not have its own unique identifier (unique index) that Access can utilize. If you do not specify the unique identifiers, you will not be able to perform updates on the linked table.

The code below needs to be run any time you want to change the server and/or database to which the tables are linked. However, if your server and/or database hasn't changed, you shouldn't need to rerun the code. In other words, you should be able to do this just prior to deploying to your users, and everything will run properly.

If you've added this code in a module, you can invoke it from the Debug Window (Ctrl-G) by typing:

FixConnections "MyServer", "MyDatabase"

Note that I'm using DAO (Data Access Objects) to make this work. Access 2000 or 2002 use ADO (ActiveX Data Objects) by default: you'll have to add a reference to DAO in order to be able to use this code. To do this, open any code module, then select Tools | References from the menu bar. Scroll through the list of available references until you find the one for Microsoft DAO 3.6 Object Library, and select it.

Note also that this is a revised version of the code. There was a problem with the previous version handling certain cases of indexes in the linked table. I've tested this as thoroughly as I can (given I wasn't having those problems), and I think it'll work now. However, I've put some specific error checking when there's a problem with the indexes. If you're posting a problem to the newsgroup, please indicate exactly what's shown in that error message.

And I should point out that you should use this code on a copy of your database. If you run into a problem, throw away that copy of the database and make a new one before you try running the code again. It's entirely possible that there might be problems if the code has run partway through, but not completed successfully.

'***************** Code Start **************

Type TableDetails
    TableName As String
    SourceTableName As String
    Attributes As Long
    IndexSQL As String
    Description As Variant
End Type

Sub FixConnections(ServerName As String, DatabaseName As String)
' This code was originally written by
' Doug Steele, MVP  djsteele@gmail.com
' You are free to use it in any application
' provided the copyright notice is left unchanged.
'
' Description:  This subroutine looks for any TableDef objects in the
'               database which have a connection string, and changes the
'               Connect property of those TableDef objects to use a
'               DSN-less connection.
'               This specific routine connects to the specified SQL Server
'               database on a specified server. It assumes trusted connection.
'
' Inputs:   ServerName:     Name of the SQL Server server (string)
'           DatabaseName:   Name of the database on that server (string)
'

On Error GoTo Err_FixConnections

Dim dbCurrent As DAO.Database
Dim prpCurrent As DAO.Property
Dim tdfCurrent As DAO.TableDef
Dim intLoop As Integer
Dim intToChange As Integer
Dim strDescription As String
Dim typNewTables() As TableDetails

  intToChange = 0

  Set dbCurrent = DBEngine.Workspaces(0).Databases(0)

' Build a list of all of the connected TableDefs and
' the tables to which they're connected.

  For Each tdfCurrent In dbCurrent.TableDefs
    If Len(tdfCurrent.Connect) > 0 Then
      ReDim Preserve typNewTables(0 To intToChange)
      typNewTables(intToChange).Attributes = tdfCurrent.Attributes
      typNewTables(intToChange).TableName = tdfCurrent.Name
      typNewTables(intToChange).SourceTableName = tdfCurrent.SourceTableName
      typNewTables(intToChange).IndexSQL = GenerateIndexSQL(tdfCurrent.Name)
      typNewTables(intToChange).Description = Null
      typNewTables(intToChange).Description = tdfCurrent.Properties("Description")
      intToChange = intToChange + 1
    End If
  Next

' Loop through all of the linked tables we found

  For intLoop = 0 To (intToChange - 1)

' Delete the existing TableDef object

    dbCurrent.TableDefs.Delete typNewTables(intLoop).TableName

' Create a new TableDef object, using the DSN-less connection

    Set tdfCurrent = dbCurrent.CreateTableDef(typNewTables(intLoop).TableName)
    tdfCurrent.Connect = "ODBC;DRIVER={sql server};DATABASE=" & _
                        DatabaseName & ";SERVER=" & ServerName & _
                        ";Trusted_Connection=Yes;"
    tdfCurrent.SourceTableName = typNewTables(intLoop).SourceTableName
    dbCurrent.TableDefs.Append tdfCurrent

' Where it existed, add the Description property to the new table.

    If IsNull(typNewTables(intLoop).Description) = False Then
      strDescription = CStr(typNewTables(intLoop).Description)
      Set prpCurrent = tdfCurrent.CreateProperty("Description", dbText, strDescription)
      tdfCurrent.Properties.Append prpCurrent
    End If

' Where it existed, create the __UniqueIndex index on the new table.

    If Len(typNewTables(intLoop).IndexSQL) > 0 Then
      dbCurrent.Execute typNewTables(intLoop).IndexSQL, dbFailOnError
    End If
  Next

End_FixConnections:
  Set tdfCurrent = Nothing
  Set dbCurrent = Nothing
  Exit Sub

Err_FixConnections:
' Specific error trapping added for Error 3291
' (Syntax error in CREATE INDEX statement.), since that's what many
' people were encountering with the old code.
' Also added error trapping for Error 3270 (Property Not Found.)
' to handle tables which don't have a description.

  Select Case Err.Number
    Case 3270
      Resume Next
    Case 3291
      MsgBox "Problem creating the Index using" & vbCrLf & _
          typNewTables(intLoop).IndexSQL, _
          vbOKOnly + vbCritical, "Fix Connections"
      Resume End_FixConnections
    Case Else
      MsgBox Err.Description & " (" & Err.Number & ") encountered", _
          vbOKOnly + vbCritical, "Fix Connections"
      Resume End_FixConnections
  End Select

End Sub

Function GenerateIndexSQL(TableName As String) As String
' This code was originally written by
' Doug Steele, MVP  djsteele@gmail.com
' You are free to use it in any application,
' provided the copyright notice is left unchanged.
'
' Description: Linked Tables should have an index __uniqueindex.
'              This function looks for that index in a given
'              table and creates an SQL statement which can
'              recreate that index.
'              (There appears to be no other way to do this!)
'              If no such index exists, the function returns an
'              empty string ("").
'
' Inputs:   TableDefObject: Reference to a Table (TableDef object)
'
' Returns:  An SQL string (or an empty string)
'

On Error GoTo Err_GenerateIndexSQL

Dim dbCurr As DAO.Database
Dim idxCurr As DAO.Index
Dim fldCurr As DAO.Field
Dim strSQL As String
Dim tdfCurr As DAO.TableDef

  Set dbCurr = CurrentDb()
  Set tdfCurr = dbCurr.TableDefs(TableName)

  If tdfCurr.Indexes.Count > 0 Then

' Ensure that there's actually an index named
' "__UnigueIndex" in the table

    On Error Resume Next
    Set idxCurr = tdfCurr.Indexes("__uniqueindex")
    If Err.Number = 0 Then
      On Error GoTo Err_GenerateIndexSQL

' Loop through all of the fields in the index,
' adding them to the SQL statement

      If idxCurr.Fields.Count > 0 Then
        strSQL = "CREATE INDEX __UniqueIndex ON [" & TableName & "] ("
        For Each fldCurr In idxCurr.Fields
          strSQL = strSQL & "[" & fldCurr.Name & "], "
        Next

' Remove the trailing comma and space

        strSQL = Left$(strSQL, Len(strSQL) - 2) & ")"
      End If
    End If
  End If

End_GenerateIndexSQL:
  Set fldCurr = Nothing
  Set tdfCurr = Nothing
  Set dbCurr = Nothing
  GenerateIndexSQL = strSQL
  Exit Function

Err_GenerateIndexSQL:
' Error number 3265 is "Not found in this collection
' (in other words, either the tablename is invalid, or
' it doesn't have an index named __uniqueindex)
  If Err.Number <> 3265 Then
    MsgBox Err.Description & " (" & Err.Number & ") encountered", _
        vbOKOnly + vbCritical, "Generate Index SQL"
  End If
  Resume End_GenerateIndexSQL

End Function

'************** Code End *****************

Addendum: I received e-mail about this from Bryan Beissel. Bryan indicated that he didn't want to use Trusted Connection. Based on information at Carl Prothman's site, he modified the routine to accept an id and password, and tried to use the following connection string:

    tdfCurrent.Connect = "ODBC;DRIVER={sql server};DATABASE=" & _
                        DatabaseName & ";SERVER=" & ServerName & _
                        ";Uid=" & myUserName & _
                        ";Pwd=" & myPassword & ";"

I haven't tried this myself (as I always use trusted connection). On first glance, it would seem to be a questionable idea, as you'd end up store the User ID and password in plain text in the Connect property. However, if that doesn't bother you, go for it!

Addendum 2: Bill Murphy pointed out in microsoft.public.access.modulesdaovba that my code wasn't propagating the Description property if one existed. D'oh!

The code above has been modified to allow for this. Note that the Description property doesn't actually exist unless a description has been added. That means that if you refer to the Description property and one doesn't exist, you'll end up with an Error (3270: Property not found.). To get around this, I'm trapping for error 3270 in my error handling in the routine FixConnections. I'm storing the Description in a Variant field, so that I can actually save a Null if no description exists. Note that I have to add the newly created TableDef object to the TableDefs collection before I can add the description. (I certainly didn't expect that!)

Addendum 3: Someone (I'm afraid I've forgotten whom) had a scenario where some of the tables were linked to another Jet database, and others were linked using ODBC to SQL Server. The code above makes the simplifying assumption that all tables are linked to the same backend. If you only need to relink some of the tables, you'll have to come up with your own approach to determining which tables need to be relinked.

For instance, if all of your ODBC-linked tables need to be changed, you can rewrite the section of code that populates typNewTables, such as:

' Build a list of all of the connected TableDefs and
' the tables to which they're connected.
  For Each tdfCurrent In dbCurrent.TableDefs
    If Len(tdfCurrent.Connect) > 0 Then
      If Left$(tdfCurrent.Connect, 5) = "ODBC;" Then
        ReDim Preserve typNewTables(0 To intToChange)
        typNewTables(intToChange).Attributes = tdfCurrent.Attributes
        typNewTables(intToChange).TableName = tdfCurrent.Name
        typNewTables(intToChange).SourceTableName = tdfCurrent.SourceTableName
        typNewTables(intToChange).IndexSQL = GenerateIndexSQL(tdfCurrent.Name)
        typNewTables(intToChange).Description = Null
        typNewTables(intToChange).Description = tdfCurrent.Properties("Description")
        intToChange = intToChange + 1
      End If
    End If
  Next

Addendum 4: While this doesn't directly affect what's discussed on this page, if you're using Vista, rather than use

    tdfCurrent.Connect = "ODBC;DRIVER={sql server};DATABASE=" & _
                        DatabaseName & ";SERVER=" & ServerName & _
                        ";Trusted_Connection=Yes;"

    tdfCurrent.Connect = "ODBC;DRIVER={SQL Native Client};DATABASE=" & _
                        DatabaseName & ";SERVER=" & ServerName & _
                        ";Trusted_Connection=Yes;"

Addendum 5: George Hepworth pointed out to me that if the connection string you provide is incorrect (eg. the Server is mistyped, or the database doesn't exist), you're going to end up deleting all your linked tables. For this reason, you might consider storing the details of the linked tables in a local table (or in the registry or in an INI file).

This page is maintained by Last Updated: 22nd December, 2009