7 DB Access Using ADO&DataEnv

8/8/2019 7 DB Access Using ADO&DataEnv

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Database Access using ADO I

(ADO Data Control &

Data Bound Control)

Vivek Gupta

IT & Systems Group

Indian Institute of Management

Lucknow

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OneADO Data Control


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ADO Data Control The ADO (ActiveX Data Object) data control is the

primary interface between a Visual Basic application

and a database. It can be used without writing any code at all! Or, It can be a central part of a complex database

management system.

The ADO data control does not appear in the standardVisual Basic toolbox - it must be added.

Select Project from the main menu, then clickComponents. The Components window will appear.

Select Microsoft ADO Data Control, then click OK. Thecontrol will be added to your toolbox.

The data control (or tool) can access databases created byother programs besides Visual Basic (or MS Access).

Some other formats supported include dBase, FoxPro,and Paradox.

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A form that uses an ADO data control

Description

AnADOdata control defines the recordsetfor aform and provides functions for navigating,

displaying, adding, and updating records.

Bound controls automatically display the valuesin the fields in the current record of the recordset

thats defined by a data control.


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Updating records with a data control

The user can change the data in any one of thebound controls.

Then, when the user clicks on one of thenavigation buttons in the data control to move to

another record, the current record is automatically

updated.

If the user closes the form before moving toanother record, though, the changes are lost.

Deleting records with a data control

Some functions, like deleting records, arent builtinto the data control.

To provide for these functions, you use VisualBasic code.

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Two

Data Bound Control

Would be revisiting ADO Data Control soon


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Data Bound Controls

Controls that are tied to database fields and

are generally not available for free-forminput.

they automatically display database fields.(Data aware controls is a synonym.)

Data-Bound controls on the form:

Set the bound controls DataSource propertyto point to the data control

set the bound controls DataFieldproperty tothe database field it is to display

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Data Bound Controls (contd) The DataChanged property is set to True whenever a bound

controls contents are changed by the user. Before ending a program, be sure to check the DataChanged

property of every bound control to see if the user madechanges.

As data is displayed in the bound controls, the

DataChanged property is set to False. If the user or any other operation changes the value in the

bound control, the DataChanged property is set to True. Simply moving to another record doesn't affect the

DataChanged property.

When the data control starts to move to a different record,the Validate event occurs.

If DataChanged is True for any bound control, the datacontrol automatically invokes the Edit and Update methods topost the changes to the database.

If you don't wish to save changes from a bound control tothe database, you can set the DataChanged property toFalse in the Validate event


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Data BoundControls Special controls with properties established by

database fields.

A data bound control is needed for each field (column)in the Recordset (database table) we need to view.

Most of the standard Visual Basic tools can be usedas data bound controls

Label : used to provide display-only access to aspecified text data field.Caption property is data bound.

Text Box: used to provide read/write access to aspecified text data field. Text property is data bound.

Check Box: Used to provide read/write access to a

Boolean field. Value property is data bound. Picture Box & Image Box: Used to display a graphical

image from a bitmap, icon, gif, jpeg, or metafile file.Provides read/write access to a image/binary data field.Picture property is data bound.

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Data BoundControls (contd) Three custom data bound controls,

data bound versions of the standard list box (DataList), thestandard combo box (DataCombo), and the standard grid control(DataGrid).

Properties: To establish the connection of the data bound control to adatabase, we use a few properties:

DataChanged: Indicates whether a value displayed in a boundcontrol has changed.

DataField: Specifies the name of a field in the table pointed to bythe respective data control.

DataSource: Specifies which data control the control is bound to(indirectly specifying the database table).

If the data in any data bound control is changed and the usermoves to another record in the database, the database willautomatically be updated with the new data (assuming it is not

ReadOnly). Be aware of this - it is an extremely powerful feature of the

data control, but also a potential source of problems.


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Data BoundControls (contd) Steps in placing the controls on a form:

Draw the bound control on the same form as the data

control to which it will be bound. Set the DataSource property. Click on the drop-down arrow

to list the data controls on your form. Choose one.

Set the DataField property. Click on the drop-down arrow to

list the fields associated with the selected data controlrecords. Make your choice.

Set all other properties, as needed.

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An Example


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Back toADO Data Control

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ADO Data Control The data control can perform the following tasks:

Connect to a database.

Open a specified database table.

Create a virtual table based on a database query.

Pass database fields to other Visual Basic tools, fordisplay or editing. Such tools are bound to thedatabase, or data bound controls.

Add new records, delete records, or update records.

Trap any errors that may occur while accessing data.

Close the database.

As a rule, you need one data control for everydatabase table, or virtual table, you need access to.

One row of the table is accessible to each data controlat any one time.

This is referred to as the current record


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ADO Data Control

When an ADO data control is placed on a form, itappears with the assigned caption and four arrowbuttons:

Move to first/previous/last/next record

The arrows are used to navigate through the tablerecords (rows).

The buttons can be used to move to the beginning ofthe table, the end of the table, or from record torecord.

In most applications, the data control never appearson the form its Visible property is almost alwaysFalse.

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ADO Data Control Properties The ADO data control is connected to a database simply

be setting a few properties. Important properties of thisdata control are: Align: Determines where data control is displayed.

Caption: Phrase displayed on the data control.

CommandType: Establishes source of Recordset (table or query). ConnectionString: Contains the information used to establish a

connection to a database.

EditMode: Read-only at run-time. Indicates current state of editing for thecurrent record.

LockType: Indicates the type of locks placed on records during editing(default setting makes databases read-only).

Recordset: A set of records defined by a data controls ConnectionString

and RecordSource properties. Run-time only.

RecordSource: Determines the table (or virtual table) the data control is

attached to Visible: Establishes whether the data control appears on the form at run-

time


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More ADO data control properties

Pro pe rty De s criptio n

UserName If a user name isnt included in the

ConnectionString property, it must bespecified for this property.

Password If a password isnt included in the

ConnectionString property, it must be

specified for this property.

EOFAction Determines what happens when the user

tries to move past the last record in the

recordset.

BOFAction Determines what happens when the usertries to move past the first record in the

recordset.

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ConnectionString Property

After placing a data control on a form, weset the ConnectionString property.

The ADO data control can connect to avariety of database types.

There are three ways to connect to adatabase: using a data link,

using an ODBC data source, or

using a connection string.


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Two dialog boxes for building a connection string

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ConnectionString Property

Visual Basic can build the connection string for us. The steps to create our example connection string

are: Start a new Visual Basic project and place an ADO Data Control on

the form. Go to the Properties Window, click on the Connection String. Click

on the ellipsis that appears. The Property Pages window appears. Choose Use Connection String and click the Build button. The

Data Link Properties window appears. Choose the Provider tab and select Microsoft Jet 4.0 OLE DB

Provider (an Access database). Click the Next button to go to the Connection tab. Click the ellipsis and use the Select Access Database dialog box to

choose the BIBLIO.MDB file in your working directory. ClickOpen.

Click Test Connection. Then, click OK (assuming it passed). ClickOK in the Property Pages window. The connection string is builtand assigned to the ConnectionString property of the data control.


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ADO Data Control Events Events that are triggered at various times during database

access, where we write code to perform specific neededtasks.

WillMove: Triggers before record pointer is moved from one row toanother.

MoveComplete: Event triggered after record pointer has been moved fromone row to another.

EndofRecordset: Triggers when the record pointer is moved past the lastrecord in the recordset.

WillChangeRecordset: Triggers before a change is made to therecordset. Used to trap unwanted changes.

RecordsetChangeComplete: Triggers after a change is made torecordset.

WillChangeRecord: Triggers before updates for the current row are sent

to the data source. RecordChangeComplete: Triggers after updates for the current row are

sent to the data source.

WillChangeField: Triggers before the current field in the recordset isupdated.

FieldChangeComplete: Triggers after the current field in the recordset

has been updated

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DAO vs. ADO ?

The primary difference between the two data

controls is in the properties used to connect to adatabase.

The DAO control connects to the database using the

DatabaseName property while the ADO control usesthe ConnectionString.

To construct a Recordset, the DAO control uses theRecordSource property. The ADO control requires

setting two properties: RecordSource andCommandType.

The ADO data control offers more event procedures

than the DAO control to allow more complete controlover database management


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ADO Recordset Object

An important concept.

When we set the RecordSource property (eitherselect a table from the database or form a virtualtable via a query), the data control (using the Jetengine) retrieves the needed records and places

them in the Recordset object for our use.

This object has its own properties and methods for our

use.

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The Recordset Object Is at the heart of every VB application that

manipulates databases.

It is a programmable object where cursors are stored Through this object, we can manipulate the rows of a

cursor, view their fields, and update the underlyingtables.

An important aspect of Recordsets is that we canchoose their location whether theyll reside on theserver, or whether theyll be downloaded to theclient.

Another important property is that it can be

disconnected from its source and we can still processit on the client, and on reconnecting to the server, wecan update the underlying tables.


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ADO Recordset Properties Can only be accessed at run-time.

To refer to a Recordset property, use a 'double-dot' notation.e.g., if you have a data control named datExample, to referto a property named PropertyName, the notation is:

datExample.Recordset.PropertyName

Important data control Recordset properties are: AbsolutePosition: Long integer that either gets or sets the

position of the current record.

BOF: Returns True when the current record is positioned beforeany data.

Bookmark: Sets or returns a bookmark to the current record.Used as a place marker in database management tasks.

EditMode: Indicates the state of editing for the current record.

EOF: Returns True when the current record is positioned past anydata.

RecordCount: The total number of records in the Recordset.

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ADO Recordset Methods Important Recordset methods are:

AddNew: Adds a new record to the Recordset. All fields are set to nulland this record becomes the current record.

CancelUpdate: Used to cancel any pending updates (either whileediting or using the AddNew method)

Close: Closes a Recordset.

Delete: The current record is deleted from the Recordset.

Move: Moves the current record pointer a specified number of recordsforward or backward.

MoveFirst: Moves the current record pointer to the first record in theRecordset.

MoveLast : Moves the current record pointer to the last record in theRecordset.

MoveNext : Moves the current record pointer to the next record in theRecordset.

MovePrevious: Moves the current record pointer to the previousrecord in the Recordset.

Requery: Updates the data in a Recordset object by re-executing thequery on which the object is based.

Update: Saves the current contents of all data bound controls


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Example: Recordset Navigation

cmdPrevious_Click event:Private Sub cmdPrevious_Click()

datTitles.Recordset.MovePreviousIf datTitles.Recordset.BOF Then

datTitles.Recordset.MoveFirst

End If

End Sub

cmdNext_Click event:Private Sub cmdNext_Click()

datTitles.Recordset.MoveNext

If datTitles.Recordset.EOF ThendatTitles.Recordset.MoveLast

End If

End Sub

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Data Environment


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ADO Data Environment

The Data Environment acts like multiple data controls wecan access from anywhere in our application.

We can connect to multiple databases and form multipleviews of the data in those databases.

Data bound controls can bind to any data view in the DataEnvironment

The Data Environment is a shareable and reusableconnection file that can be used in any VB project. It is added to our project just like a form

(it has a single property of interest - Name).

The Data Environment provides a greatly simplifiedprogramming environment used to connect to datasources. It provides 'drag-and-drop' functionality for building

interfaces and developing database reports

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ADO Data Environment

The Data Environmenthas two primary objects:

the Connection object

specifies the database

similar to theConnectionStringproperty of the ADO

data control, and

the Command object

specifies theRecordset like theRecordSourceproperty of the ADO

control.


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The Connection Object

Establishes a connection to a database, which is thenused to execute commands against the database orretrieve a Recordset Its most important property is the ConnectionString

property, which specifies the database we want toconnect to.

To establish a connection through a Connection object,call its Open method

Use Close method to close this connection when notneeded

To find out whether a connection is open, use its Stateproperty

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Connection Object

It specifies the information needed to connect to aparticular database.

The Connection object has several properties:

Attributes: Needed connection string properties

passed to the Connection object.

CommandTimeout: Time in seconds the server will

wait for a command to return a reply.

ConnectionSource: String that describes the path to

the data source connection.

ConnectionTimeout: Time in seconds the server willwait for a connection to open on the destination server


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Command Object The Command object specifies a database table

(either a native table or a virtual table formed using

a database query). Some Command object properties are:

CommandText: Specifies table to display or givesvalid database query statement.

CommandType: Specifies whether object isconnected to database native table or virtual tableformed using a query.

ConnectionName: Name of associated Connection

object. LockType: Controls how data in table may or may not

be changed. Set to Read Only by default. Must bechanged if you need to edit data.

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Example: Use of ADO Data Environment

Here, we will form a Connection object tothe books database (BIBLIO.MDB) databaseand a Command object for the Titles table.

This should illustrate the differences betweenusing the Data Environment and the ADOdata control


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Data bound controls with Data Environment

To establish the connection of a data boundcontrol to a database using the DataEnvironment, we use three properties:DataField: Specifies the name of a field in the

table established by the Command object.

DataMember: Specifies the Command objectestablishing the database table.

DataSource: Specifies which Data

Environment the control is bound to. NB: there is one more property needed when

compared to using the ADO data control -DataMember is not used with the data control

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The relationships between a data boundcontrol (DataSource, DataMember andDataField properties) and the Data

Environment are:


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To use bound controls with the Data Environment,

steps are:

Draw the bound control on the form.

Set the DataSource property. Click on the drop-downarrow to list the Data Environments in your project.Choose one.

Set the DataMember property. Click on the drop-

down arrow to list the Command objects in theselected Data Environment. Choose one.

Set the DataField property. Click on the drop-downarrow to list the fields associated with the selected

DataMember. Make your choice.

Set all other properties, as needed

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Drag and Drop Data Bound Controls

One of the powerful features of the DataEnvironment

Recordsets in the Data Environment

the Data Environment returns a Recordset that hasproperties and methods identical to those of the ADO

data control.

A Recordset is returned for each Command object in theenvironment.

Each possible recordsets in one Data Environment is

assigned a unique name by the environment, based onthe Command object name.

Make sure you give each Command object in a DataEnvironment a unique name.


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Example: Use of Drag & Drop

Here, we will build the same interface withthe ADO data control using the drag & dropcapabilities off the Data Environment.

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Recordsets in the Data Environment

For a Command object named comExample,the Recordset is assigned the namerscomExample (note the addition of the two

letter prefix, rs). To read or set the property of this Recordsetin a Data Environment named denExample,we use the double-dot notation of:

denExample.rscomExample.PropertyName

Likewise to reference a method (such as oneof the Move methods) for this Recordset, we

use the notation: denExample.rscomExample.MethodName


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Example: Record Navigation with D.EN.

Continuing with previous eg., we will add twocommand buttons

cmdPrevious_Click event:Private Sub cmdPrevious_Click()

denBooks.rscomTitles.MovePrevious

If denBooks.rscomTitles.BOF Then

denBooks.rscomTitles.MoveFirst

End If

End Sub

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The Data Environment Designer window


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How to use the Designer window

To open the Designer window, you can select MoreActiveX Designers from the Project menu and then select

Data Environment.

When the Designer window is opened for the first time fora project, one DataEnvironment object and one

Connection object are shown.

To change the name of the DataEnvironment object, selectit in either the Designer or Project Explorer window, then

change the name in the Properties window.

To change the way the objects are arranged in the DataEnvironment Designer window, click on the Arrange By

Connections or the Arrange By Objects toolbar buttons. To remove a DataEnvironment object from a project,

select the object in the Project Explorer window and chose

the Remove command from either the Project menu or the

shortcut menu.

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The Provider and Connection tabs of the Data LinkProperties dialog box


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How to edit or create a Connection object

To edit a Connection object, select that object in theDesigner window and click on the Properties button in

the Designer toolbar.

To create a new Connection object, click on the AddConnection button in the Designer toolbar.

When the Data Link Properties dialog box is displayed,you must complete the Properties and Connection tabs

as you did in chapters 6 and 7.

When you add a Connection object to the Designerwindow, its also added to the Data View window

that's presented in chapter 11. However, you have to

use the Designer window, not the Data View window,to edit the properties of the Connection object or to see

its Command objects.

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The General tab of the Propertiesdialog box for a Command object


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How to create a Command object

To create a new Command object, click on the AddCommand button on the Designer toolbar. This displays the

General tab of the Properties dialog box for the Command

object. From the General tab, enter a name for the command and

choose one of the Connection objects that has been created

for the DataEnvironment object.

Then, if you click on the Database Object button, you canselect Stored Procedure, Table, or View from the first drop-

down list and the stored procedure, table, or view that you

want to use as the data source from the Object Name list.

Or, if you click on the SQL Statement button, you can enteran SQL statement as the data source.

To build an SQL statement for a command using a graphicalinterface, you can click on the SQL Builder button.

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A bound form created from a Command object


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How to build a form from a Command object

To create a bound form from a Command object, drag theobject and drop in onto the form.

To add individual fields to a form, expand the Command object

in the Designer window so it shows all the fields defined by theobject. Then, drag-and-drop one field name at a time onto the

form.

If you look at the Data properties for a bound control that hasbeen added to the form in this way, you will see that the

DataSource property is set to the designer name (deAP), the

DataMember property is set to the command name (Vendors),

and the DataField property is set to the field name.

After you add the controls to the form, you can move or editthem in the usual ways.

To complete a form, you need to add navigation controls thatlet the user move from one record to another.

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The Options and Field Properties dialog boxes

Description

You can use the Options dialog box to change the generaldefaults for the way controls are added to a form.

You can use the Field Properties dialog box to change themapping default or caption for a single field.


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A completion list for a DataEnvironment object

Statements that refer to properties and methodsdeAP.rsVendors.RecordCount

deAP.rsVendorInvoices.EOF

deAP.rsVendors.MoveFirst

deAP.rsVendorInvoices.Find

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The Query Designer window for a query thatretrieves data from two tables


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How to use the Query Designer

When you're creating or editing a Command object, you can use the QueryDesigner to generate the SQL query that you need. To do that, click the

SQL Builder button on the General tab of the Properties dialog box for a

Command object.

When the Query Designer starts, the Data View window that's described inchapter 11 is also opened. Then, to add tables to the query, you drag them

from the Data View window and drop them in the diagram pane of the

Query Designer.

To include a field in the query and add it to the grid pane, check its box inthe diagram pane. To include all the fields in a table, check the All

Columns box. To include a calculated field, enter an expression in the

Column column. To sort the records based on one or more fields, choose Ascending or

Descending from the drop-down list in the Sort Type column.

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How to use the Query Designer (continued)

To select the records to be included in the results, enter criteria in theCriteria and Or columns of the grid pane. If you want to use a field to

select records but you dont want to include it in the results, remove the

check mark from the Output column for the field.

To execute the SQL statement that's shown in the SQL pane, select the

Run command from the Query menu or the shortcut menu. If the queryreturns a recordset, the data is displayed in the results pane. Otherwise,

youll see a message indicating how many records were affected by the

query.

To save the SQL statement in the Command object, close the QueryDesigner window and click Yes in the resulting dialog box.

To resize a pane, drag its top or bottom boundary. To hide or show apane, use the Hide Pane command in the shortcut menu or the Show

Panes submenu of the View menu.


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Return to ADO

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ThreeADO Object Model


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The ActiveX Data Object (ADO) model

Connection

Recordset

Fields Field

Command

Parameters

Errors Error

Parameter

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ADO concepts

The Connection object represents a connection toa data source.

A Recordset object represents a set of recordsreturned from a query.

You can use the Fields collection of a Recordsetobject to refer to specific fields in the recordset.

A collection is simply a set of objects of the sametype.

A Command object contains the definition of acommand, such as a SQL statement, the name of

a table, or the name of a stored procedure.

You can use the Parameters collection of aCommand object to create parameter queries or to

specify arguments for a stored procedure.


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ADO concepts (continued)

The Errors collection contains one or more Errorobjects that contain information about the most

recent errors that occurred in response to an ADO

operation.

You can use these objects to get moreinformation than the Visual Basic Err object

provides.

The ADO model lets you access a data sourceusing an object-oriented interface.

That means you can use the same syntax to

access data objects that you use to access objectslike forms and controls.

The ADO model also provides a variety ofproperties, methods, and events for working with

the ActiveX data objects.

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How does ADO work ?

ADO is a component for accessing databases

It exposes a few very flexible objects, which expose

their own properties, methods, and recognize events

The structure of the ADO object model reflects theoperations we perform on a database,

We can even guess these objects and their names oncewe understand what it takes to access and update adatabase.

First step to using ADO is to connect to a database

This connection object is our gateway to the database


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Common ADO properties

Property Object

ConnectionString Connection

Provider ConnectionDefaultDatabase Connection

CursorLocation Connection

Recordset

BOF

EOF

Recordset

CursorType RecordsetLockType Recordset

RecordCount Recordset

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Common ADO methods

Method Object

Open Connection

Recordset

AddNew Recordset

Update Recordset

Delete Recordset

MoveFirst

MoveLast

MovePrevious

MoveNext

Recordset

Requery Recordset

Close ConnectionRecordset


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Typical declarations for ADO variablesDim cnAP As Connection

Dim rsVendors As Recordset

Typical assignment statements for ADOvariablesSet cnAP = New Connection

Set rsVendors = New Recordset

Set rsVendors = Nothing

Typical statements for working withADO variablesrsVendors.Open "Vendors",cnAP,adOpenKeyset, _

adLockOptimistic,adCmdTabletxtName = rsVendors!VendorName

If rsVendors.EOF Then rsVendors.MoveLast

rsVendors.Close

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Description Use the Dim statement (Private or Public) to declare an object

variable for an ADO object

After above declaration, we use the Set statement to create

an instanceof the object and assign a reference to theobject to the variable.

To do this, we use the New keyword and the name of the ADOobject

After creating an instance of an object, we can refer to its

properties, methods, and events in the code.

To disassociate a variable from the object it refers to, assign avalue of Nothing to the value. Then, the system resources

associate with the object are released.

To refer to a field in a recordset, code the recordset name, the

bang operator (!), and the field name as in, rsVendors!VendorName


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A procedure that deletes a record from the datasource associated with a data control

Private Sub cmdDelete_Click()If MsgBox("Do you want to delete this record?", _

vbYesNo + vbDefaultButton2 + vbQuestion) _

= vbYes Then

adoVendors.Recordset.Delete

adoVendors.Recordset.MoveNext

If adoVendors.Recordset.EOF Then

adoVendors.Recordset.Requery

adoVendors.Recordset.MoveLast

End If

End If

End Sub

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UnderstandingCursors & Recordsets


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Cursors & Recordsets

There is a direct relationship between SQL Server cursors andVB Recordsets. Every time we issue a SELECT statement, SQL Server

retrieves the qualifying rows and returns them to the caller,which is a stored procedure or a Visual Basic application.

This set of rows is called a cursor

When the cursor is passed to the VB application, its stored in aRecordset object.

The cursor contains the info. retrieved from the DB, the rawinformation.

A Recordset is an ADO object that stores these rows. It containsthe functionality we need to access these rows in the form of

properties and methods. A recordset is simply an object that references a cursor. It exposes a number of methods and properties we can use

to navigate the cursor (not the Recordset) and edit the rowsof the underlying cursor.

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Cursors & Recordsets

A typical application sets up a connection to adatabase and executes commands through thisconnection, but it spends most of its time processingthe rows returned by the DBMS

After establishing a connection to the database andpreparing the command object, we can execute thecommand and retrieve the cursor. However, wemust prepare an object where the cursor will bestored. This object is a Recordset variable, that must be

declared as: Dim RS as ADODB.Recordset

Then execute the command with theExecute methodand assign the cursor returned to the RS variable:

Set RS = Cmd.Execute


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Types of Cursors Different applications have different requirements,

which get reflected by the cursor-related properties

as well as by the LockType property Cursors are categorized according to their type

(CursorType property) and where they reside(CursorLocation property)

The type of cursor determines how fresh the cursoris, i.e., whether the application can see changes madeto the cursor by other users or not.

If the cursor resides on the client,then its type is static

Other types apply to cursors that reside on server.

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CursorLocation options

Option Des cription

adUseClient The cursor and its associated result set

reside on the client.

adUseServer The cursor and its associated result set

reside on the server.

Note

If we use client-side cursors (adUseClient), Visual Basic

automatically sets the cursor type to adOpenStatic without

notifying us, no matter what we set the CursorType to.


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CursorType options

Option De s cription

adOpenDynamic Opens a dynamic cursor, which includes

changes, additions, and deletions made to

the database by other users.

Uses keys to identify individual records.

Cannot be used to create sorted result sets

from SQL Server data.

adOpenKeyset Opens a keyset cursor, which includes

changes made to the database by other

users.

Additions made by other users arentvisible, but records deleted by other users

are no longer accessible.

Uses keys to identify individual records.

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CursorType options (continued)

Option Description

adOpenStatic Opens a static cursor, which doesnt

include changes, additions, and deletionsmade to the database by other users.

This is the only type of cursor that can be

created on the client with ADO.

adOpenForwardOnly Opens a forward-only cursor.

This cursor is like a static cursor, but youcan only scroll forward through the

records.

This type of cursor isnt available when

you define a recordset using a data control.


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Three

ADO Saga Continues

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LockType options

Option Des cription

adLockReadOnly Data can be viewed, but not modified.

When used with SQL Server data, this

causes a shared lock to be placed on the

records so other users can access and

change the data at the same time.adLockPessimistic Causes the records to be locked

immediately so other users cant change

them.

When used with SQL Server data, this

causes an update lock to be placed on the

records when theyre retrieved. The

update lock is upgraded to an exclusive

lock when the records are actuallyupdated.


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LockType options (continued)

adLockOptimistic Causes the records to be locked when

theyre updated.

When used with SQL Server data, thiscauses a shared lock to be placed on the

records when theyre retrieved. Then,

when the Update method is issued, theshared lock is upgraded to an update lock.

And when the records are actually

updated, the update lock is upgraded to an

exclusive lock.

adLockBatchOptimistic Used for updating records in batchesinstead of one at a time.

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An Unbound form using code to maintain vendor records

Description

The navigation buttons are an array of command buttons.

When a record is first displayed, the Update button is disabled. As soonas the user makes a change to the data in the form, however, the Update

button is enabled and the Add, Delete, and navigation buttons are

disabled.

To save the changes, the user must click on the Update button or pressthe Enter key to activate the Update button.


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Description (continued)

To cancel the changes, the user can press the Esc key.

When the user clicks on the Add button, the controls arecleared, the Update button is enabled, and the Add,

Delete, and navigation buttons are disabled.

After the data for the new record is entered into theform, the user can save the record by clicking on the

Update button or pressing the Enter key.

To cancel the addition, the user can press the Esc key.

If the user clicks on the Update button to add a newrecord or modify an existing record and a required field

hasnt been entered, a message box is displayed

identifying the field. When the user presses the Delete key, a dialog box is

displayed to confirm the delete operation. If the user

confirms the operation, the record is deleted and the

next record in the recordset is displayed.

78

Functions that are provided through code

Function Metho d or

statement

When exe cuted

Open connection Open method When the form is loaded

Open recordset Open method When the form is loaded

Recordset

navigation

MoveFirst,

MovePrevious,

MoveNext,

MoveLast

When the user clicks on a

navigation button or

when other events require

this movement

Move values

from fields to

controls

Assignment

statements

When the recordset is

opened, when the user

clicks on a navigation

button, when a record is

deleted, and when

changes to a record arecanceled


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More functions that are provided through code

Fun c tio n Me th o d o r

s ta tem e nt

W h e n e x e c u te d

Clear control

values

Assignment

statements

When the user wants to

add a record

Add a new

record to the

recordset

AddNew

method

When the user wants to

add a record

Move values

from controls to

current record,

and update the

recordset

Assignment

statements,

Update method

When the user indicates

that recordset should be

updated with the data for

a new record or the

changes to an old recordClose a

connection or

recordset

Close m ethod W hen the connection or

recordset is no longer

needed

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Open method syntax for a Connection objectconnection.Open [ConnectionString][, UserID][, Password]

ConnectionString arguments for SQL Server

Arg ume nt De s criptio n

Provider The name of the OLE DB provider

(SQLOLEDB)

Server/Data

Source

The name of the server where the database

resides

Database/

Initial Catalog

The name of the database

UID/UserID The user name to use when connecting to

the data source

PWD/Password The password to use when connecting to thedata source


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Statements that open a connection to a SQLServer databaseDim cnAP As Connection

Set cnAP = New Connection

cnAP.CursorLocation = adUseClient

cnAP.Open "Provider=SQLOLEDB;Server=mmaserver;" _

& "Database=AccountsPayable", "ed", "abc4xyz"

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Open method syntax for a Recordset objectrecordset.Open [Source][, ActiveConnection][, CursorType]

[, LockType][, Options]

Argume nt Des c ription

Source A SQL statement, the name of a table, or

the name of a stored procedure.

ActiveConnection A valid Connection object or string.

CursorType The type of cursor to be used for the

recordset

LockType The type of locking to be used for the

recordset

Options The type of information specified in the

Source argument


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Statements that open a recordset with a SELECTstatement and a Connection objectDim rsVendors As Recordset, strSQL As String

Set rsVendors = New RecordsetstrSQL = "SELECT * FROM Vendors ORDER BY VendorName"

rsVendors.Open strSQL, cnAP, adOpenKeyset, _

adLockOptimistic, adCmdText

Statements that open the same recordset using aconnection string and no Connection objectDim strConnection As String

strConnection = "Provider=SQLOLEDB;Server=mmaserver;" _

& "UID=ed;PWD=abc4xyz;Database=AccountsPayable"

rsVendors.Open strSQL, strConnection, adOpenKeyset, _adLockOptimistic, adCmdText

84

The properties for two controls in an array

Description

To create a control array, you add two or more controls ofthe same type to the form.

Then, you change the names of the controls so theyre all the

same and Visual Basic sets the Index property of eachcontrol so its unique.


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How to use a control array

To refer to a control in an array, you use the controlname followed by its index number in parentheses,

starting with zero for the first control in the array as inthis example:

cmdNavigate(0)

The controls in a control array are treated as a group. Sowe code a single event procedure for the entire array

When an event occurs on a control in an array, VisualBasic passes an argument to the event procedure that

contains the index of the control that the event occurred

on. Then, you can use that index to determine the

processing thats done.

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Code for working with the control arrayPrivate Sub cmdNavigate_Click(Index As Integer)

Select Case Index

Case 0

rsVendors.MoveFirst

Case 1

rsVendors.MovePrevious

If rsVendors.BOF Then rsVendors.MoveFirst

Case 2

rsVendors.MoveNext

If rsVendors.EOF Then rsVendors.MoveLast

Case 3

rsVendors.MoveLast

End Select

End Sub


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Code for the unbound Vendors form

Option Explicit 1

Dim mcnAP As Connection

Dim mrsVendors As Recordset

Dim mbAddNew As BooleanConst msDialogTitle = "Chapter 6 - Vendors (Unbound)"

Private Sub Form_Load() 2

Set mcnAP = New Connection

Set mrsVendors = New Recordset

mcnAP.Open "Provider=SQLOLEDB;Server=mmaserver;" _

& "Database=AccountsPayable", "ed",

"abc4xyz"

mrsVendors.Open _

"SELECT * FROM Vendors ORDER BY VendorName", _

mcnAP, adOpenKeyset, adLockOptimistic, adCmdTextLoadControls

SetCommandButtons True

SetNavigationButtons True

End Sub

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Private Sub LoadControls() 3

With mrsVendors

txtName = !VendorName

txtAddress1 = !VendorAddress1 & ""

txtAddress2 = !VendorAddress2 & ""

txtCity = !VendorCity

txtState = !VendorState

txtZipCode = !VendorZipCodetxtContact = !VendorContact & ""

txtPhone = !VendorPhone & ""

End With

End Sub

Private Sub SetCommandButtons(bVal As Boolean) 4

cmdAdd.Enabled = bVal

cmdUpdate.Enabled = Not bVal

cmdUpdate.Default = Not bVal

cmdDelete.Enabled = bVal

End Sub


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Private Sub SetNavigationButtons(bVal As Boolean) 5Dim i As Integer

For i = 0 To 3

cmdNavigate(i).Enabled = bVal

Next

End Sub

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Private Sub cmdNavigate_Click(Index As Integer) 6

With mrsVendors

Select Case Index

Case 0

.MoveFirst

Case 1

.MovePrevious

If .BOF Then .MoveFirstCase 2

.MoveNext

If .EOF Then .MoveLast

Case 3

.MoveLast

End Select

End With

LoadControls



txtName.SetFocusEnd Sub


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Private Sub cmdAdd_Click() 7

mbAddNew = True

ClearControls

SetCommandButtons FalsetxtName.SetFocus

End Sub

Private Sub ClearControls() 8

txtName = ""

txtAddress1 = ""

txtAddress2 = ""

txtCity = ""

txtState = ""

txtZipCode = ""

txtContact = ""

txtPhone = ""

End Sub

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Private Sub cmdUpdate_Click() 9

On Error GoTo ErrorHandler

If ValidData Then

If mbAddNew Then mrsVendors.AddNew

LoadRecord

mrsVendors.Update

mbAddNew = False



txtName.SetFocus

End If

Exit Sub

ErrorHandler:

DisplayErrorMsg

If mrsVendors.EditMode = adEditAdd Then

mrsVendors.CancelUpdate

End Sub


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Private Function ValidData() As Boolean 10

Dim strMessage As String

If txtName = "" Then

txtName.SetFocus

strMessage = "You must enter a vendor name."

ElseIf txtCity = "" Then

txtCity.SetFocus

strMessage = "You must enter a city."

ElseIf txtState = "" Then

txtState.SetFocus

strMessage = "You must enter a state code."

ElseIf txtZipCode = "" Then

txtZipCode.SetFocus

strMessage = "You must enter a zip code."

Else

ValidData = True

End IfIf Not ValidData Then

MsgBox strMessage, vbOKOnly, msDialogTitle

End If

End Function

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Private Sub LoadRecord() 11

With mrsVendors

!VendorName = txtName

!VendorAddress1 = txtAddress1

!VendorAddress2 = txtAddress2

!VendorCity = txtCity

!VendorState = txtState

!VendorZipCode = txtZipCode

!VendorContact = txtContact

!VendorPhone = txtPhone

End With

End Sub


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Private Sub cmdDelete_Click() 12

On Error GoTo ErrorHandler

If MsgBox("Do you want to delete this record?", _

vbYesNo + vbDefaultButton2 + vbQuestion, _

msDialogTitle) _

= vbYes Then

With mrsVendors

.Delete

.MoveNext

If .EOF Then

.Requery

.MoveLast

End If

LoadControls

End With

End If


SetNavigationButtons TruetxtName.SetFocus

Exit Sub

ErrorHandler:

DisplayErrorMsg

End Sub

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Private Sub txtName_Change() 13

SetNavigationButtons False

SetCommandButtons False

End Sub

Private Sub txtAddress1_Change()

SetNavigationButtons FalseSetCommandButtons False

End Sub

.

.

.

Private Sub txtPhone_Change()

SetNavigationButtons False

SetCommandButtons False

End Sub


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Private Sub Form_KeyDown(KeyCode As Integer, _ 14

Shift As Integer)

If KeyCode = vbKeyEscape Then

LoadControls



End If

End Sub

Private Sub cmdExit_Click() 15

mrsVendors.Close

mcnAP.Close

Set mrsVendors = Nothing

Set mcnAP = Nothing

Unload frmVendors

End

End Sub

Private Sub DisplayErrorMsg() 16

MsgBox "Error Code: " & Err.Number & vbCrLf & _

"Description: " & Err.Description, _

vbOKOnly + vbCritical, msDialogTitle

End Sub

When bound forms are best

When the forms are relatively simple

When the built-in functions are acceptable to the users

When you need to build an application quickly without fullfunctionality

When unbound forms are best When the forms are relatively complex

When the user interface needs to work in a way that can't beaccomplished with bound forms

When you can't get the bound forms to do what you want them todo without serious debugging problems (see chapter 7)

Notes

7 DB Access Using ADO&DataEnv

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