CHAPTER 21
Using Access as a back end to enhance multiuser access to data

The example near the end of Chapter 19, “Text file processing,” proposes a method for storing 660,601,620 records in an Excel worksheet. At some point, you need to admit that even though Excel is the greatest product in the world, there is a time to move to Access and take advantage of Access multidimensional database (MDB) files.

Even before you have more than 1 million rows, another compelling reason to use MDB data files is to allow multiuser access to data without the headaches associated with shared workbooks.

Microsoft Excel offers an option to share a workbook, but you automatically lose a number of important Excel features when you do this type of sharing. After you share a workbook, you cannot use automatic subtotals, pivot tables, Group and Outline mode, scenarios, protection, or the Styles, Pictures, Add Charts, and Insert Worksheets options.

By using an Excel VBA front end and storing data in an MDB database, you have the best of both worlds. You have the power and flexibility of Excel and the multiuser access capability available in Access.

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 1048576 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:=AdForwardOnly, 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 the information down 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:=AdForwardOnly, _

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.

Checking for the existence of tables

If the application in this chapter’s example needs a new table in the database, you can use the code in the next section. However, because you have a multiuser application, only the first person who opens the application has to add the table on the fly. When the next buyer shows up, the table might have already been added by the first buyer’s application. Because this code is a function instead of a sub, it returns either True or False to the calling routine.

This code uses the OpenSchema method to query the database schema:

Click here to view code image

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

Checking for the existence of a field

Sometimes you want to add a new field to an existing table. The following code does this, and it uses the OpenSchema method but this time looks at the columns in the tables:

Click here to view code image

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

Adding a table on the fly

The following code uses a pass-through query to tell Access to run a Create Table command:

Click here to view code image

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

Adding a field on the fly

If you determine that a field does not exist, you can use a pass-through query to add a field to the table, like this:

Click here to view code image

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.

Click here to view code image

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

Click here to view code image

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.