Visual Basic ADO Programming 56:150 Information System Design

Introduction 1 Microsoft ActiveX Data Objects (ADO) enables you to write an application to access and manipulate data in a database server through an OLE DB data provider. High speed, ease of use, low memory overhead, and a small disk footprint

Introduction 2 What’s data provider A control or object that provides data for use with another control or program. The data provider makes data connectivity much easier by hiding most of the implementation of data storage. What’s OLE DB A set of COM-based interfaces provide applications with uniform access to data stored in diverse information sources, or data stores

Introduction 3 To use ADO objects in an application, you must first add a reference to the ADO component. Start a Standard EXE project and then select Project  References. In the Reference window, locate Microsoft ActiveX Data Objects 2.x Library and check the box before it.

Main Objects The ADO object model defines a collection of programmable objects that can be used by any of the Microsoft Visual languages

The Connection Object to establish connections between the client and database server ConnectionString Property a long string with several attributes separated by semicolons “Provider = Microsoft.Jet.OLEDB.4.0; Data Source=C:\Program Files\VB98\Nwind.mdb” “Provider=SQLOLEDB.1;User ID=sa; password=; Initial Catalog=Northwind; Data Source=EXPERTNEW”

The Connection Object Open Method CN.open The open method accepts a number of optional arguments (ConnString, UserID, password, options) Close Method CN.Close Set CN = Nothing (remove the Connection Object from memory)

Connection Example Dim dbcon as ADODB.Connection Set dbcon = New ADODB.Connection dbcon.ConnectionString _ ="Provider=MSDASQL.1;Persist Security _ Info=False;Data Source=NWIND" dbcon.ConnectionTimeout = 10 dbcon.Open dbcon.close Set dbcon = Nothing

The Command Object to issue commands, such as SQL queries and updates, to the database ActiveConnection Property If ActiveConnection is set with a reference to a Connection Object, the Command object uses an exiting connection. If ActiveConnection is set with a connection string, a new connection is established.

The Command Object Execute Method Use the Execute method of the Command object to execute a query, data definition command, or stored procedure. Set rs = cmd.Execute(NumRecords, Parameters, Options) Options specify the type of query (in the form of CommandTypeEnum constant) to optimize processing.

CommandTypeEnum adCmdStoreProc The command is the name of a Stored procedure adCmdTable The command is a table’s name. “Select * from table_name” is passed to the server adCmdTableDirect The command is a table’s name. More efficient that adCmdTable option adCmdText The command is a SQL statement adCmdUnknown The command is unknown (default)

Command Example Dim cmd As ADODB.Command Dim rst As ADODB.Recordset Set cmd = New ADODB.Command cmd.CommandText = "select distinct ShipCountry from orders" cmd.CommandType = adCmdText Set cmd.ActiveConnection = dbcon Set rst = New ADODB.Recordset Set rst = cmd.Execute

Command Example You can do delete, update, insert using Command Object with the right sql sentence. Dim cmd As ADODB.Command Dim lngAffected As Integer Set cmd = New ADODB.Command cmd.ActiveConnection = dbcon cmd.CommandType = adCmdText cmd.CommandText = "UPDATE tblOrders SET ShipCountry = 'United States' WHERE ShipCountry = 'USA'“ cmd.Execute lngAffected

The Recordset Object to view and manipulate the results of the query Open Method To execute a query open ( [Source], [ActiveConnection], [CursorType As CursorTypeEnum = adOpenUnspecified], [LockType As LockTypeEnum = adLockUnspecified], [Options As Long = -1])) Source can be a sql statement, a valid command object, a table name, a query name (Access), a stored procedure name (SQL Server) Options is a constant that indicates how the provider should evaluate the Source argument if it represents something other than a Command object

Cursor Type (CursorTypeEnum) adOpenForwardOnlyThis cursor can be scanned forward only, is suitable for one-pass operations. Less expensive than other types of cursors (default) adOpenStatic A snapshot of the database the moment the cursor was created. It can be scanned in both directions. You can’t see modifications made by other users after the creation of the cursor. adOpenKeyset Like a dynamic cursor, except that you can't see records that other users add. Data changes by other users are still visible. adOpenDynamic Additions, changes, and deletions by other users are visible, and all types of movement through the Recordset are allowed.

The Recordset Object Example Dim rst As ADODB.Recordset Dim StrSQL As String Set rst = New ADODB.Recordset StrSQL = "select Description from categories where categoryname = '" & Combocategory.Text & "'" rst.Open Source:=StrSQL, ActiveConnection:=dbcon, Options:=adCmdText

The Recordset Object AddNew: add new rows to recordset rst.AddNew rst.Fields("LastName") = "Smith" rsr.Fields("FirstName") = "Tommy" rst.Update Use the update method to save the new row. If you attempt to close the recordset with an update pending but haven't explicitly saved the row, you'll get a runtime error

The Record Object Change data Move to the desired row Make changes optionally use update method to save updates rst.Find "[ContactTitle] = 'Owner'" If rst.EOF Then MsgBox "No Match was Found!" Else rst.Fields("ContactTitle") = "Manager" rst.Update

The Recordset Object Delete records Find the desired rows Use delete method to delete. rst.Find "[ContactTitle] = 'Owner'" If rst.EOF Then MsgBox "No Match was Found!" Else rst.delete End if

The Recordset Object Other frequently used methods Cancelupdate, Movefirst, Movenext, Movelast, Moveprevious Other frequently used Properties Fields, Filter, RecordCount

ADO Data Control Nothing new but a wrapper for the ADO Recordset object. Unlike the Recordset object, ADODC is visible at run time. It will be shown in Sample program.