ADO versus DAO

For several years, Microsoft recommended using data access objects (DAOs) for accessing data in an external database. DAOs became very popular, and a great deal of code was written for them. When Microsoft released Excel 2000, it started pushing ActiveX Data Objects (ADOs). The concepts are similar, and the syntax differs only slightly. I use ADO in this chapter. Realize that if you start going through code written a decade ago, you might run into DAO code. Other than a few syntax changes, the code for both ADO and DAO looks similar.

To use any code in this chapter, open the VB Editor. Select Tools, References from the main menu and then select Microsoft ActiveX Data Objects Library from the Available References list, as shown in Figure 21-1.

The remainder of this chapter gives you the code necessary to allow the application included in the preceding case study to read or write data from the tblTransfer table.

The tools of ADO

You encounter several terms when using ADO to connect to an external data source:

• Record set—When connecting to an Access database, the record set is either a table in the database or a query in the database. Most of the ADO methods reference the record set. You might also want to create your own query on the fly. In this case, write a SQL statement to extract only a subset of records from a table.

• Connection—The connection defines the path to the database and the type of database. In the case of Access databases, you specify that the connection is using the Microsoft Jet Engine.

• Cursor—Think of the cursor as a pointer that keeps track of which record you are using in the database. There are several types of cursors and two places for the cursor to be located (described in the following bullets).

• Cursor type—A dynamic cursor is the most flexible cursor. If you define a record set and someone else updates a row in the table while a dynamic cursor is active, the dynamic cursor knows about the updated record. Although this is the most flexible, it requires the most overhead. If your database doesn’t have a lot of transactions, you might specify a static cursor; this type of cursor returns a snapshot of the data at the time the cursor is established.

• Cursor location—The cursor can be located either on the client or on the server. For an Access database residing on your hard drive, a server location for the cursor means that the Access Jet Engine on your computer is controlling the cursor. When you specify a client location for the cursor, your Excel session is controlling the cursor. On a very large external data set, it would be better to allow the server to control the cursor. For small data sets, a client cursor is faster.

• Lock type—The point of this chapter is to allow multiple people to access a data set at the same time. The lock type defines how ADO will prevent crashes when two people try to update a record at the same time. With an optimistic lock type, an individual record is locked only when you attempt to update the record. If your application will be doing 90% reads and only occasionally updating, then an optimistic lock is perfect. However, if you know that every time you read a record, you will soon update the record, you should use a pessimistic lock type. With pessimistic locks, a record is locked as soon as you read it. If you know that you will never write back to the database, you can use a read-only lock. This enables you to read the records without preventing others from writing to them.

The primary objects needed to access data in an MDB file are an ADO connection and an ADO record set.

The ADO connection defines the path to the database and specifies that the connection is based on the Microsoft Jet Engine.

After you have established the connection to the database, you usually use that connection to define a record set. A record set can be a table or a subset of records in the table or a predefined query in the Access database. To open a record set, you have to specify the connection and the values for the CursorType, CursorLocation, LockType, and Options parameters.

Assuming that you have only two users trying to access the table at a time, you should use a dynamic cursor and an optimistic lock type. For large data sets, the adUseServer value of the CursorLocation property allows the database server to process records without using up RAM on the client machine. If you have a small data set, it might be faster to use adUseClient for the CursorLocation. When the record set is opened, all the records are transferred to memory of the client machine. This allows faster navigation from record to record.

Reading data from the Access database is easy, provided that you have fewer than 1,048,576 records. You can use the CopyFromRecordset method to copy all selected records from the record set to a blank area of the worksheet.

To add a record to an Access table, use the AddNew method for the record set. You then specify the value for each field in the table and use the Update method to commit the changes to the database.

To delete a record from the table, you can use a pass-through query to delete records that match a certain criteria.

Other tools are available that let you make sure that a table exists or that a particular field exists in a table. You can also use VBA to add new fields to a table definition on the fly.

Adding a record to a database

Going back to the case study earlier in the chapter, the application you are creating has a userform where buyers can enter transfers. To make the calls to the Access database as simple as possible, a series of utility modules handle the ADO connection to the database. This way, the userform code can simply call AddTransfer(Style, FromStore, ToStore, Qty).

Here’s how you add records after the connection is defined:

1. Open a record set that points to the table. In the code that follows, see the sections commented ' Open the Connection, ' Define the Recordset, and ' Open the Table.

2. Use AddNew to add a new record.

3. Update each field in the new record.

4. Use Update to update the record set.

5. Close the record set and then close the connection.

The following code adds a new record to the tblTransfer table:

Click here to view code image

Sub AddTransfer(Style As Variant, FromStore As Variant, _
 ToStore As Variant, Qty As Integer)
 Dim cnn As ADODB.Connection
 Dim rst As ADODB.Recordset
 
 MyConn = "J:	ransfers.mdb"
 
 ' Open the Connection
 Set cnn = New ADODB.Connection
 With cnn
   .Provider = "Microsoft.Jet.OLEDB.4.0"
   .Open MyConn
 End With
 
' Define the Recordset
 Set rst = New ADODB.Recordset
 rst.CursorLocation = adUseServer
 
 ' Open the Table
 rst.Open Source:="tblTransfer", _
   ActiveConnection:=cnn, _
   CursorType:=adOpenDynamic, _
   LockType:=adLockOptimistic, _
   Options:=adCmdTable
 
 ' Add a record
 rst.AddNew
 
 ' Set up the values for the fields. The first four fields
 ' are passed from the calling userform. The date field
 ' is filled with the current date.
 Rst("Style") = Style
 rst("FromStore") = FromStore
 rst("ToStore") = ToStore
 rst("Qty") = Qty
 rst("tDate") = Date
 rst("Sent") = False
 rst("Receive") = False
 
 ' Write the values to this record
 rst.Update
 
 ' Close
 rst.Close
 cnn.Close
 
End Sub

Retrieving records from a database

Reading records from an Access database is easy. As you define a record set, you pass a SQL string to return the records you are interested in.

After the record set is defined, use the CopyFromRecordSet method to copy all the matching records from Access to a specific area of the worksheet.

The following routine queries the Transfer table to find all records in which the Sent flag is not yet set to True:

Click here to view code image

Sub GetUnsentTransfers()
 Dim cnn As ADODB.Connection
 Dim rst As ADODB.Recordset
 Dim WSOrig As Worksheet
 Dim WSTemp As Worksheet
 Dim sSQL as String
 Dim FinalRow as Long
 
 Set WSOrig = ActiveSheet
 
 'Build a SQL String to get all fields for unsent transfers
 sSQL = "SELECT ID, Style, FromStore, ToStore, Qty, tDate " _
 & "FROM tblTransfer"
 sSQL = sSQL & " WHERE Sent=FALSE"
 
 ' Path to Transfers.mdb
 MyConn = "J:	ransfers.mdb"
 
 Set cnn = New ADODB.Connection
 With cnn
   .Provider = "Microsoft.Jet.OLEDB.4.0"
   .Open MyConn
 End With
 
 Set rst = New ADODB.Recordset
 rst.CursorLocation = adUseServer
 rst.Open Source:=sSQL, ActiveConnection:=cnn, _
    CursorType:=AdOpenForwardOnly, LockType:=adLockOptimistic, _
    Options:=adCmdText
 
 ' Create the report in a new worksheet
 Set WSTemp = Worksheets.Add
 
 ' Add Headings
 Range("A1:F1").Value = Array("ID", "Style", "From", "To", "Qty", "Date")
 
 ' Copy from the record set to row 2
 Range("A2").CopyFromRecordset rst
 
 ' Close the connection
 rst.Close
 cnn.Close
 
 ' Format the report
 FinalRow = Range("A65536").End(xlUp).Row
 
 ' If there were no records, then stop
 If FinalRow = 1 Then
    Application.DisplayAlerts = False
    WSTemp.Delete
    Application.DisplayAlerts = True
    WSOrig.Activate
    MsgBox "There are no transfers to confirm"
    Exit Sub
 End If
 
 ' Format column F as a date
 Range("F2:F" & FinalRow).NumberFormat = "m/d/y"
 
 ' Show the userform – used in next section
 frmTransConf.Show
 
 ' Delete the temporary sheet
 Application.DisplayAlerts = False
 WSTemp.Delete
 Application.DisplayAlerts = True
 
End Sub

The results are placed on a blank worksheet. The final few lines display the results in a userform to illustrate how to update a record in the next section.

The CopyFromRecordSet method copies records that match the SQL query to a range on the worksheet. Note that you receive only the data rows. The headings do not come along automatically. You must use code to write the headings to row 1. Figure 21-3 shows the results.

Updating an existing record

To update an existing record, you need to build a record set with exactly one record. This requires that the user select some sort of unique key when identifying the records. After you have opened the record set, use the Fields property to change the field in question and then the Update method to commit the changes to the database.

The earlier example returned a record set to a blank worksheet and then called the userform frmTransConf. This form uses a simple Userform_Initialize to display the range in a large list box:

Click here to view code image

Private Sub UserForm_Initialize()
 
 ' Determine how many records we have
 FinalRow = Cells(Rows.Count, 1).End(xlUp).Row
 If FinalRow > 1 Then
   Me.lbXlt.RowSource = "A2:F" & FinalRow
 End If
 
End Sub

The list box’s properties have the MultiSelect property set to True.

After the Userform_Initialize procedure is run, the unconfirmed records are displayed in a list box. The logistics planner can mark all the records that have been sent, as shown in Figure 21-4.

The code attached to the Confirm button follows:

Click here to view code image

Private Sub cbConfirm_Click()
 Dim cnn As ADODB.Connection
 Dim rst As ADODB.Recordset
 
 ' If nothing is selected, warn them
 CountSelect = 0
 For x = 0 To Me.lbXlt.ListCount - 1
 If Me.lbXlt.Selected(x) Then
   CountSelect = CountSelect + 1
 End If
 Next x
 
 If CountSelect = 0 Then
 MsgBox "There were no transfers selected. " & _
   "To exit without confirming any transfers, use Cancel."
 Exit Sub
 End If
 
 ' Establish a connection to transfers.mdb
 ' Path to Transfers.mdb is on Menu
 MyConn = "J:	ransfers.mdb"
 
 Set cnn = New ADODB.Connection
 
 With cnn
   .Provider = "Microsoft.Jet.OLEDB.4.0"
   .Open MyConn
 End With
 
 ' Mark as complete
 For x = 0 To Me.lbXlt.ListCount - 1
 If Me.lbXlt.Selected(x) Then
   ThisID = Cells(2 + x, 1).Value
   ' Mark ThisID as complete
   'Build SQL String
    sSQL = "SELECT * FROM tblTransfer Where ID=" & ThisID
   Set rst = New ADODB.Recordset
   With rst
     .Open Source:=sSQL, ActiveConnection:=cnn, _
       CursorType:=adOpenKeyset, LockType:=adLockOptimistic
     ' Update the field
     .Fields("Sent").Value = True
     .Update
     .Close
     End With
      End If
   Next x
 
 ' Close the connection
 cnn.Close
 Set rst = Nothing
 Set cnn = Nothing
 
 ' Close the userform
 Unload Me
 
End Sub

Including the ID field in the fields returned in the prior example is important if you want to narrow down the information to a single record.

Deleting records via ADO

As with updating a record, the key to deleting records is being able to write a bit of SQL to uniquely identify the records to be deleted. The following code uses the Execute method to pass the Delete command through to Access:

Click here to view code image

Public Sub ADOWipeOutAttribute(RecID)
 ' Establish a connection to transfers.mdb
 MyConn = "J:	ransfers.mdb"
 
 With New ADODB.Connection
      .Provider = "Microsoft.Jet.OLEDB.4.0"
      .Open MyConn
      .Execute "Delete From tblTransfer Where ID = " & RecID
      .Close
 End With
End Sub

Summarizing records via ADO

One strength of Access is running summary queries that group by a particular field. If you build a summary query in Access and examine the SQL view, you’ll see that complex queries can be written. Similar SQL can be built in Excel VBA and passed to Access via ADO.

The following code uses a fairly complex query to get a net total by store:

Click here to view code image

Sub NetTransfers(Style As Variant)
 ' This builds a table of net open transfers
 ' on Styles AI1
 Dim cnn As ADODB.Connection
 Dim rst As ADODB.Recordset
 
 ' Build the large SQL query
 ' Basic Logic: Get all open Incoming Transfers by store,
 ' union with -1* outgoing transfers by store
 ' Sum that union by store, and give us min date as well
 ' A single call to this macro will replace 60
 ' calls to GetTransferIn, GetTransferOut, TransferAge
 sSQL = "Select Store, Sum(Quantity), Min(mDate) From " & _
   "(SELECT ToStore AS Store, Sum(Qty) AS Quantity, " & _
   "Min(TDate) AS mDate FROM tblTransfer where Style='" & Style & _
   "& "' AND Receive=FALSE GROUP BY ToStore "
 sSQL = sSQL & " Union All SELECT FromStore AS Store, " & _
   "Sum(-1*Qty) AS Quantity, Min(TDate) AS mDate " & _
   "FROM tblTransfer where Style='" & Style & "' AND " & _
   "Sent=FALSE GROUP BY FromStore)"
 sSQL = sSQL & " Group by Store"
 
 MyConn = "J:	ransfers.mdb"
 
 ' open the connection.
 Set cnn = New ADODB.Connection
 With cnn
   .Provider = "Microsoft.Jet.OLEDB.4.0"
   .Open MyConn
 End With
 
 Set rst = New ADODB.Recordset
 
 rst.CursorLocation = adUseServer
 
 ' open the first query
 rst.Open Source:=sSQL, _
    ActiveConnection:=cnn, _
    CursorType:=AdOpenForwardOnly, _
    LockType:=adLockOptimistic, _
    Options:=adCmdText
 
 Range("A1:C1").Value = Array("Store", "Qty", "Date")
 ' Return Query Results
 Range("A2").CopyFromRecordset rst
 rst.Close
 cnn.Close
 
End Sub

Other utilities via ADO

Consider the application you created for this chapter’s case study: The buyers now have an Access database located on their network but possibly no copy of Access. It would be ideal if you could deliver changes to the Access database on the fly as their application opens.

Function TableExists(WhichTable)
 Dim cnn As ADODB.Connection
 Dim rst As ADODB.Recordset
 Dim fld As ADODB.Field
 TableExists = False
 
 ' Path to Transfers.mdb is on Menu
 MyConn = "J:	ransfers.mdb"
 
 Set cnn = New ADODB.Connection
 
 With cnn
 .Provider = "Microsoft.Jet.OLEDB.4.0"
 .Open MyConn
 End With
 
 Set rst = cnn.OpenSchema(adSchemaTables)
 
 Do Until rst.EOF
    If LCase(rst!Table_Name) = LCase(WhichTable) Then
       TableExists = True
       GoTo ExitMe
    End If
    rst.MoveNext
 Loop
 
ExitMe:
 rst.Close
 Set rst = Nothing
 ' Close the connection
 cnn.Close
 
End Function

Function ColumnExists(WhichColumn, WhichTable)
 Dim cnn As ADODB.Connection
 Dim rst As ADODB.Recordset
 Dim WSOrig As Worksheet
 Dim WSTemp As Worksheet
 Dim fld As ADODB.Field
 ColumnExists = False
 
 ' Path to Transfers.mdb is on menu
 MyConn = ActiveWorkbook.Worksheets("Menu").Range("TPath").Value
 If Right(MyConn, 1) = "" Then
   MyConn = MyConn & "transfers.mdb"
 Else
   MyConn = MyConn & "	ransfers.mdb"
 End If
 
 Set cnn = New ADODB.Connection
 
 With cnn
    .Provider = "Microsoft.Jet.OLEDB.4.0"
    .Open MyConn
 End With
 
 Set rst = cnn.OpenSchema(adSchemaColumns)
 
 Do Until rst.EOF
 If LCase(rst!Column_Name) = LCase(WhichColumn) And _
    LCase(rst!Table_Name) = LCase(WhichTable) Then
    ColumnExists = True
    GoTo ExitMe
   End If
 rst.MoveNext
 Loop
 
ExitMe:
   rst.Close
   Set rst = Nothing
   ' Close the connection
   cnn.Close
 
End Function

Sub ADOCreateReplenish()
 ' This creates tblReplenish
 ' There are five fields:
 ' Style
 ' A = Auto replenishment for A
 ' B = Auto replenishment level for B stores
 ' C = Auto replenishment level for C stores
 ' RecActive = Yes/No field
 Dim cnn As ADODB.Connection
 Dim cmd As ADODB.Command
 
 ' Define the connection
 MyConn = "J:	ransfers.mdb"
 
 ' open the connection
 Set cnn = New ADODB.Connection
 With cnn
   .Provider = "Microsoft.Jet.OLEDB.4.0"
   .Open MyConn
 End With
 
 Set cmd = New ADODB.Command
 Set cmd.ActiveConnection = cnn
 'create table
 cmd.CommandText = "CREATE TABLE tblReplenish " & _
    "(Style Char(10) Primary Key, " & _
    "A int, B int, C Int, RecActive YesNo)"
 cmd.Execute , , adCmdText
 Set cmd = Nothing
 Set cnn = Nothing
 Exit Sub
End Sub

Sub ADOAddField()
 ' This adds a grp field to tblReplenish
 Dim cnn As ADODB.Connection
 Dim cmd As ADODB.Command
 
 ' Define the connection
 MyConn = "J:	ransfers.mdb"
 
 ' open the connection
 Set cnn = New ADODB.Connection
 With cnn
    .Provider = "Microsoft.Jet.OLEDB.4.0"
    .Open MyConn
 End With
 
 
 Set cmd = New ADODB.Command
 Set cmd.ActiveConnection = cnn
 'create table
 cmd.CommandText = "ALTER TABLE tblReplenish Add Column Grp Char(25)"
 cmd.Execute , , adCmdText
 Set cmd = Nothing
 Set cnn = Nothing
 
End Sub

SQL Server examples

If you have 64-bit versions of Office and if Microsoft does not provide the 64-bit Microsoft.Jet.OLEDB.4.0 drivers, you have to switch over to using SQL Server or another database technology:

Click here to view code image

Sub DataExtract()
 
Application.DisplayAlerts = False
 
'clear out all previous data
Sheet1.Cells.Clear
 
' Create a connection object.
Dim cnPubs As ADODB.Connection
Set cnPubs = New ADODB.Connection
 
' Provide the connection string.
Dim strConn As String
 
'Use the SQL Server OLE DB Provider.
strConn = "PROVIDER=SQLOLEDB;"
 
'Connect to the Pubs database on the local server.
strConn = strConn & "DATA SOURCE=a_sql_server;INITIAL CATALOG=a_database;"
 
'Use an integrated login.
strConn = strConn & " INTEGRATED SECURITY=sspi;"
 
'Now open the connection.
cnPubs.Open strConn
 
' Create a record set object.
Dim rsPubs As ADODB.Recordset
Set rsPubs = New ADODB.Recordset
 
With rsPubs
    ' Assign the Connection object.
    .ActiveConnection = cnPubs
    ' Extract the required records.
    .Open "exec a_database..a_stored_procedure"
    ' Copy the records into cell A1 on Sheet1.
    Sheet1.Range("A2").CopyFromRecordset rsPubs
Dim myColumn As Range
'Dim title_string As String
Dim K As Integer
For K = 0 To rsPubs.Fields.Count - 1
     'Sheet1.Columns(K).Value = rsPubs.Fields(K).Name
    'title_string = title_string & rsPubs.Fields(K).Name & Chr(9)
    'Sheet1.Columns(K).Cells(1).Name = rsPubs.Fields(K).Name
    'Sheet1.Columns.Column(K) = rsPubs.Fields(K).Name
    'Set myColumn = Sheet1.Columns(K)
    'myColumn.Cells(1, K).Value = rsPubs.Fields(K).Name
    'Sheet1.Cells(1, K) = rsPubs.Fields(K).Name
    Sheet1.Cells(1, K + 1) = rsPubs.Fields(K).Name
    Sheet1.Cells(1, K + 1).Font.Bold = "TRUE"
Next K
'Sheet1.Range("A1").Value = title_string
 
 
   ' Tidy up
   .Close
End With
 
cnPubs.Close
Set rsPubs = Nothing
Set cnPubs = Nothing
 
'clear out errors
Dim cellval As Range
Dim myRng As Range
Set myRng = ActiveSheet.UsedRange
For Each cellval In myRng
    cellval.Value = cellval.Value
    'cellval.NumberFormat = "@" 'this works as well as setting
    'HorizontalAlignment
    cellval.HorizontalAlignment = xlRight
Next
 
End Sub

Next steps

In Chapter 22, “Advanced userform techniques,” you’ll discover more controls and techniques you can use in building userforms.