Accessing Data Sources with Visual Basic 6.0 William R. Vaughn Microsoft Technical Education (MSTE) Developer Trainer 12

Accessing Data Sources Accessing Data Sources with Visual Basic 6.0with Visual Basic 6.0

William R. VaughnWilliam R. VaughnMicrosoft Technical Education (MSTE)Developer Trainer

12

William R. VaughnWilliam R. VaughnMicrosoft Corporation

Author:Hitchhiker’s Guide to Visual Basic

and SQL Server Microsoft Press, 1996, 1997, 1998

[email protected]

AgendaAgenda

Enterprise Development Strategies– SQL Server 7.0 – Microsoft Database Engine (MSDE)

Making best use of Visual Basic 6.0 Tools– DataEnvironment Designer – Data Object Wizard

Implementing ADO Solutions– Effective Queries– Processing Updates– Managing Stored Procedures

Enterprise Development Strategies:Enterprise Development Strategies:SQL Server 7.0SQL Server 7.0

Larger (8000 byte) page size– Longer Char, VarChar– Less need to record BLOBs in the DB

Unicode (DBCS) support– Nchar, NVarChar, Ntext– Better international support

New Substring functions– CONTAINS, FREETEXT

Better BLOB handling– Fetch starting at “nth” byte


Top [n] [Percent [n] ]– Preferred over RowCount– Can be used in subqueries

Enhanced local cursor support New uniqueidentifier data type for

storing a globally unique identifier (GUID)– Instead of Identity datatype– Great for disjoint server updates


Automatically manages admin functions No need to:

– Update statistics– Tune procedure cache– Run DBCC maintenance routines– Preallocate disk space (no disk inits)– Stretch TempDB, DB or Log space

Automatically runs backups New GUI tools


Shares memory, disk space– Stretches or shrinks resources to match

demand

Does not create TempDB SPs– Not needed to leverage existing query plans

Shares ad-hoc and stored procedures– Executing queries now re-entrant– Automatically senses reusable queries

SQL Server 6.5 SQL Server 6.5 Cache managementCache management

Proced

ure C

ache

1st. instance is loaded

2nd. instance loaded

3rd. instance re-uses first1st. instance finishes

SQL ServerStored Procedures

SQL Server 7.0 SQL Server 7.0 Cache managementCache management

Man

aged M

emory

1st. instance is loaded

2nd. instance shares first

3rd. instance shares first1st. instance finishes

SQL ServerStored Procedures

Data Pages

SQL Server 7.0 SQL Server 7.0 Ad hoc cachingAd hoc caching

If current SQL batch matches (exactly), SQL Server 7.0 uses cached plan.

Query #3 uses cached plan from #1, but #2 is recompiled…

Query1: Insert MyTable values(1,0)Query1: Insert MyTable values(1,0)Query2: Insert MyTable values(2,0)Query2: Insert MyTable values(2,0)Query3: Insert MyTable values(1,0)Query3: Insert MyTable values(1,0)

Q1: Select * from employees where emp_id = 1000Q1: Select * from employees where emp_id = 1000Q2: Select * from employees where emp_id = 5 Q2: Select * from employees where emp_id = 5 Q3: Select * from employees where emp_id = 1000Q3: Select * from employees where emp_id = 1000

Auto parameterizationsAuto parameterizations

Engine guesses constants are parameters Similar parameters share same plan

All three use the same cached plan created with Q1.

Intelligent CachingIntelligent Caching

Tips:– Know what the parameters are?

Mark them (?)Build parameters collection (in RDO/ADO)

– Continue to use SPsThey just run faster

– SPs are now re-entrantWhich makes them even faster

Enterprise Development Strategies:Enterprise Development Strategies:SQL Server Desktop EditionSQL Server Desktop Edition

Microsoft SQL Server Desktop Edition Runs on Windows 95, 98, 2000,

NT 4.0 Workstation/Server Duplicates functionality in core

NT SQL Server 7.0 (no OLAP, EQ, FTS etc.) Performance “purposefully degraded” ~ 5

Users, Supports (only) sockets (TCP/IP) Supplied via

– SQL Server 7.0 NT versions

Includes SQL Enterprise Manager tools– Same as those that ship for NT version

Enterprise Development Strategies:Enterprise Development Strategies:MSDEMSDE

Microsoft Database Engine (MSDE) Runs on Windows 95, 98, 2000, NT 4.0

WS/Svr Duplicates functionality in core

NT SQL Server 7.0 (no OLAP, EQ, FTS etc.)

Performance “purposefully degraded” Database limited to 2GB Supplied through

– Download/CD (now)

– Office 2000


Visual Basic 6.0 “Plus” Pack– Includes SQL Server Developer Edition

To setup databasesTo create permissionsPerform administrative functions

No “Setup.EXE” (requires batch operation)

No DB setup tools—SQLDMO samples– Get tools with SQL Server Desktop (on CD)– Download 45-Day “Trial” version of SQL Server

Desktop Ed.– Get CreateDatabase tool from www.betav.com


Supports (only) sockets (TCP/IP)—no named pipes

Freely distributable (with a Visual Studio or Office 2000 app)

Does not replace Microsoft Jet Database Engine (Jet)

Requires SQL Server 7.0 to run in “per-seat” licensing mode.

See – www.msdn.microsoft.com/vstudio/msde– www.betav.com

Replicated DBReplicated DB


New standard “portable” SS7 DBMS No need to use DAO and RDO/ADO Same features, limited capacity Same tools, interfaces, programming model Easily scale up to SQL Server 7.0 NT Write once--run “anywhere”

Making best use of Making best use of Visual Basic 6.0 Tools: DEDVisual Basic 6.0 Tools: DED

Overview of DataEnvironment Technology

Introducing the Data View Window Introducing the DataEnvironment Object Solving problems with the DE

Designers vs WizardsDesigners vs Wizards

Designers– RDO: UserConnection Designer– ADO: DataEnvironment Designer– Use dialogs to capture property settings– Create a binary .DSR file– Require a runtime– Construct data access COM objects at runtime

Wizards– Ask you a series of questions– Generate Visual Basic (editable) code

DataEnvironment TechnologyDataEnvironment Technology

What is the “DE”?– Data Environment Designer created for VS 6.0– GUI tool for ADO development

What does it do?– DE Creates layer over ADO object interface

How do I use it?– Exposed as programmable “data source”– Bindable to data-aware

(ADO) controls

How can it help me?– Increases productivity– Builds team standard interface

Introducing the Data View WindowIntroducing the Data View Window

What is the Data View Window?– Data Links persisted in Windows registry

Where is it exposed?– All Visual Studio tools

What is it used for?– Capture ADO Connection properties– Create DataEnvironment objects– View/create/modify:

schema, table(s), data, indexes, relationships, view(s), stored procedures, parameters…

Functionality based on provider

Understanding the Understanding the Data View WindowData View Window

• Schema Diagrams

• Tables

• Views

• Stored Procedures

• (Project) DE Connections

New DE

New Data Link

Creating a Data LinkCreating a Data Link

What’s a “Data Link”?– A persisted connection like a “DSN”

How do you create one?– Start Visual Basic 6.0– Click “Add New Data Link” icon– Fill in Connection properties

How do I prevent extra dialogs?– Do not simply point to DSN

for SQL Server or Oracle– Use ODBC or OLE DB provider

Adding tables with a Adding tables with a Data LinkData Link

Create, Open DB Diagram

Right-click to – add table– add related tables– arrange tables

Fill in or copy/paste column properties

Drag “foreign” key to “primary” key

Introducing the Introducing the DataEnvironment ObjectDataEnvironment Object

DataEnvironment persisted with VB project Saved as .DSR file

– Loadable by other team members

Masks ADO object model– DE contains one or more Connections– Connections contain one or more Commands– Commands have parameters, properties,

hierarchies

Managed via ADO properties, methods, events

Understanding the Understanding the DataEnvironment ObjectDataEnvironment Object

Why should you use the DE?– To eliminate code–just like the UserConnection

Designer– To generate queries and stored procedures– To “correctly” generate parameters – To expose complex queries to the entire team

Where should you use the DE?– On the client–not really suitable for the middle tier– Where you want to increase developer productivity

Acts as “data source” control– Other controls can bind to DE– Position current row via rsxxx “Move” methods

Creating a Creating a DataEnvironment ObjectDataEnvironment Object

Create new “Data” project Use Data Link icon

Next... Right-click Project menu Fill in Connection properties

Setting DataEnvironment Setting DataEnvironment Connection PropertiesConnection Properties

Reference Properties - Connection window

Set properties not exposed in DE “wizard”– Command/

ConnectionTimeout– Cursor location (server/client)– Passwords (design/runtime)– Prompt behavior– Run/

DesignSaveAuthentication

Using the Data EnvironmentUsing the Data Environment

Build Data Link with Data View Window Create DataEnvironment object Expand tables/views/stored procedures Drag to DE Set properties Tune query Drag to form Provide navigation controls, code Provide parameter controls, code

Accessing ADO with the Accessing ADO with the DataEnvironment ObjectDataEnvironment Object

DE object internals Exposed as ADO objects

Drag tables/views/SPs from DE Connections lists

DataEnvironment Object DataEnvironment Object exposes ADO objectsexposes ADO objects

DataEnvironment objectDataEnvironment object

Expenses “query”Expenses “query”

CostCost “field” “field”

ADO objectADO object

DataEnvironment1DataEnvironment1 Connection Connection

ExpensesExpenses method/Command method/Command

Recordset Field objectRecordset Field object

rsExpensesrsExpenses Recordset RecordsetBindable result setBindable result set

Expenses.Parameters(.)Expenses.Parameters(.)ParametersParameters

Binding to Data Aware ControlsBinding to Data Aware Controls

Setting default controls by datatype– DataEnvironment

“Options /Field Mapping”

– Set control type for each datatype

Using the DOW to build UserControls

Binding Options:Binding Options:DataGridDataGrid

Binding Options:Binding Options:HierarchicalFlex GridHierarchicalFlex Grid

Binding Options:Binding Options:Bound ControlsBound Controls

DataEnvironment QueriesDataEnvironment QueriesTips and TechniquesTips and Techniques

Never use SELECT * Always use WHERE clause Choose the “right” (lightest weight)

cursor Limit rows to as few as possible

– Never over 200, usually < 100– Fetch More as needed

Update via stored procedure Use asynchronous options

Hierarchical QueriesHierarchical Queries

In use everywhere– Customer-order-item

– System-component-subsystem-part

– State-Town-Street

DE uses ADO/OLE DB “Shape” provider

SHAPE {SELECT * FROM "dbo"."AuthorsView"}SHAPE {SELECT * FROM "dbo"."AuthorsView"}AS AuthorsView AS AuthorsView APPEND ({SELECT Title, Year_Published, ISBN, APPEND ({SELECT Title, Year_Published, ISBN, CoverFilename FROM Titles} AS titles RELATE ) CoverFilename FROM Titles} AS titles RELATE ) AS titlesAS titles

Hierarchical QueriesHierarchical Queries

Observe “default” behavior with SQL Trace

SELECT * FROM "dbo"."Authors" SELECT * FROM "dbo"."Authors" SELECT * FROM "dbo"."Title_Author" SELECT * FROM "dbo"."Title_Author" SELECT * FROM "dbo"."Titles"SELECT * FROM "dbo"."Titles"

Be sure to limit focus of queries with views or focused queriesPassing in parameters is tough... But getting easier with ADO 2.1

Stored ProceduresStored Procedures

Creating Debugging Installing

into the DE Passing

parameters Rebinding

Creating Stored ProceduresCreating Stored Procedures

Use Data View window– Create Stored Procedure– Design (against existing SP)

Refresh DE after changes TSQL Debugger available to test Data Object Wizard (DOW) Use SQL Enterprise Manager … Use Visual InterDev Data Tools Code by hand

Installing SPs into the DEInstalling SPs into the DE

Drag from Data View window (DE explorer)

Insert Command under Connection– Set Command type– Point to SP

Set properties, but not parameter Value

Passing Parameters toPassing Parameters to Stored Procedures Stored Procedures

Don’t set in Data Environment Command properties...– Becomes a permanent part of query

Set while invoking Command as DE method Recordset passed as new rsXXXXX object

Set de = DataEnvironment1Set de = DataEnvironment1de.AuthorsByYearBorn Text1.Text, Text2.textde.AuthorsByYearBorn Text1.Text, Text2.textShowADOData rsAuthorsByYearBornShowADOData rsAuthorsByYearBorn

Passing Parameters toPassing Parameters toStored ProceduresStored Procedures

Set in WillExecute event Parameter name depends on how query

is parsed. Note: this event called twice...

Private Sub Connection1_WillExecute...Private Sub Connection1_WillExecute... pCommand("@YearLow") = Form1.Text1pCommand("@YearLow") = Form1.Text1 pCommand("@YearHigh") = Form1.Text2pCommand("@YearHigh") = Form1.Text2End SubEnd Sub

Passing Parameters toPassing Parameters to Stored Procedures (Subsequently) Stored Procedures (Subsequently)

No “Requery” available. Must Close and re-execute query … and rebind

With DataEnvironment1With DataEnvironment1 .rsAuthorsByYearBorn.Close.rsAuthorsByYearBorn.Close .AuthorsByYearBorn Text1, Text2.AuthorsByYearBorn Text1, Text2End WithEnd WithRebindControls Me, "TextBox,",RebindControls Me, "TextBox,", "AuthorsByYearBorn", "AuthorsByYearBorn", DataEnvironment1 DataEnvironment1

Rebinding after Rebinding after Rebuilding RecordsetRebuilding Recordset

Sub RebindControls(FormName, ControlType As String, CommandName, DE)

Dim ctl As Control

For Each ctl In FormName.Controls

If InStr(ControlType & ",", TypeName(ctl) & ",") Then

If ctl.DataMember = CommandName Then

Set ctl.DataSource = DE

End If

End If

Next

End Sub

Recreating ADO Recordset requires rebinding to “simple” bound controls

“Complex” bound controls handled automatically

Coding Move MethodsCoding Move Methods

Private Sub MoveNextButton_click()Private Sub MoveNextButton_click()On Error Resume NextOn Error Resume Next With DataEnvironment1.rsAuthorsByYearBornWith DataEnvironment1.rsAuthorsByYearBorn .MoveNext.MoveNext If .EOF ThenIf .EOF Then If Not .BOF Then .MoveLastIf Not .BOF Then .MoveLast End IfEnd If End WithEnd WithEnd SubEnd Sub

Default “simple” bound control binding does not include navigation controls

No drag-on control as in Visual InterDev

Part IIPart II

Making best use of Visual Basic 6.0 Wizards…

Data Object WizardData Object Wizard

Building procedure-based applications Constructing Procedures with Visual

Basic version 6.0 Building DOW-based Data classes Building DOW-generated UserControls

Building procedure-based applicationsBuilding procedure-based applications

Why “procedure-based” designs?– Higher performance– Centralized management– Higher security

Defining the problemDefining the problem

Data retrieval by key– Parameter-driven query– From any ADO data source

Resultsets displayed, manipulated by – Row – Grid – ComboBox

Updateable via procedure– Stored procedures– User-written ADO / DE Commands

UserControl

Data Object Wizard Data FlowData Object Wizard Data Flow

Fetch Query

Update Query

Dat

a S

erve

r

Data Class

Constructing the DOW proceduresConstructing the DOW procedures

Data retrieval...by key or “all” Update...by key Delete...by key Insert… by key or using Identity Can substitute Update with Delete /

Insert SQL Server? Remember your triggers Data lookup procedure

Data Retrieval ProcedureData Retrieval Procedure

Create Procedure Publisher_Fetch Create Procedure Publisher_Fetch @StateWanted varchar(10)@StateWanted varchar(10)

AsAsif @StateWanted = 'all' if @StateWanted = 'all' Begin Begin Select * from publishers Select * from publishers where State is not nullwhere State is not nullEndEndElseElseBeginBegin Select * from Publishers Select * from Publishers where State = @StateWanted where State = @StateWanted and State is not nulland State is not nullEndEndReturnReturn

Data Retrieval ProcedureData Retrieval Procedure

Returns rows based on primary key fetch (Requires) parameter query Limit scope of query to about 100 rows Pass parameter via

– UserControl property– FormLoad or Fetch procedure

Run automatically unless ManualInitialize=true

Do not use last field to fetch lookup value

Data Update ProcedureData Update Procedure

Create Procedure Publisher_Update Create Procedure Publisher_Update @PubID Int,@Name Varchar(50),@Company_Name varchar(255), @PubID Int,@Name Varchar(50),@Company_Name varchar(255), @Address varchar(50), @City varchar(20), @Address varchar(50), @City varchar(20), @State varchar(15),@Zip Varchar(15), @Telephone @State varchar(15),@Zip Varchar(15), @Telephone varchar(15),@Fax Varchar(15)varchar(15),@Fax Varchar(15)AsAsUpdate PublishersUpdate Publishersset Name= @Name, Company_Name=@Company_Name, set Name= @Name, Company_Name=@Company_Name, Address=@Address, City=@City, State=@State, Address=@Address, City=@City, State=@State, Zip=@Zip, Telephone=@TelePhone, Zip=@Zip, Telephone=@TelePhone, Fax=@Fax Fax=@Fax where PubID = @PubIDwhere PubID = @PubIDreturnreturn

Data Update ProcedureData Update Procedure

Updates row based on primary key match

Setup with parameter query All columns passed in--except Primary

Key Executed automatically when bound

data changes DOW Option to use Delete/Insert

instead

Data Delete ProcedureData Delete Procedure

Create Procedure Publisher_Delete Create Procedure Publisher_Delete (@PubID Integer)(@PubID Integer)AsAsDelete Publishers where PubID = @PubIDDelete Publishers where PubID = @PubIDreturnreturn

Data Delete ProcedureData Delete Procedure

Deletes row based on primary key match

Setup with parameter query Executed automatically when

– Bound data row is deleted– Or before insert when no update procedure

exists

Data Insert ProcedureData Insert Procedure

Create Procedure Publisher_Insert Create Procedure Publisher_Insert

@Name varchar(50), @Company_Name varchar(255), @Name varchar(50), @Company_Name varchar(255),

@Address varchar(50), @City varchar(20), @Address varchar(50), @City varchar(20),

@State varchar(15), @Zip varchar(15),@State varchar(15), @Zip varchar(15),

@Telephone varchar(15), @Fax varchar(15)@Telephone varchar(15), @Fax varchar(15)

AsAs

INSERT INTO PublishersINSERT INTO Publishers

( Name, Company_Name, Address, City, ( Name, Company_Name, Address, City,

State, Zip, Telephone, Fax)State, Zip, Telephone, Fax)

VALUES ( @Name, @Company_Name, @Address, VALUES ( @Name, @Company_Name, @Address,

@City, @State, @Zip, @Telephone, @Fax)@City, @State, @Zip, @Telephone, @Fax)

return @@identityreturn @@identity

Data Insert ProcedureData Insert Procedure

Inserts a new row via parameter query Procedure can return new @@Identity

rowID Executed automatically when

– Bound data row is added– Row is updated and no update procedure exists

DemonstrationDemonstration

Creating Stored Procedures in Visual Basic version 6.0

Connecting to the data sourceConnecting to the data source

Create New Data Environment (DE)

Delete existing Connection1

Drag Procedures to DE from Data View window

(or) Insert SPs No, don’t “Execute”

procedures

Building Procedure-based Building Procedure-based Data ClassesData Classes

Start Visual Basic 6.0 with “Data” project Install DOW Addin Build Data Class Point to Procedures,

Queries

Tip: Use SQLTrace or SQL Profiler to see what ADO, DE, and DOW are doing...

Building DOW-based Data ClassesBuilding DOW-based Data Classes

Tell DOW how to fetch individual rows– Point to fetch procedure (PublisherFetch)


Tell DOW which field(s) contain the unique key– Set Primary Key on Select procedure


Tell DOW how to insert, update, delete rows– Point to change procedures


Tell DOW how to name your DataClass– Overlays existing class… permanently

Building DOW-Generated UserControlsBuilding DOW-Generated UserControls

Point to DOW-generated Data class Choose Single Row, Data Grid, Combo

Boxes Choose VB control type for each field For Single Row and Combo Box add...

– Row navigation code– Add, delete buttons– Parameter capture code

Install UserControlInstall UserControl

Provide parameter for fetch query– Set UserControl parameter property– Via Code and Init<Data class> method– Set ManualInitialize property as required

Uct<cls>DataGrid1.<parmname> = <parm source>Uct<cls>DataGrid1.<parmname> = <parm source>uct<cls>DataGrid1.Init<cls>uct<cls>DataGrid1.Init<cls>

uctPublishersDataGrid1.dbo_Publisher_FetchStateWanted = StateWanted.TextuctPublishersDataGrid1.dbo_Publisher_FetchStateWanted = StateWanted.TextuctPublishersDataGrid1.InitPublishersuctPublishersDataGrid1.InitPublishers

Set GridEditable = TrueRun it!


Creating a simple DOW-based UserControl

Adding a Lookup dropdownAdding a Lookup dropdown

Lookup valid entries for selected fields Substitute underlying coded entries

– In displayed UserControl columns– In Inserted, Updated procedures

Create Lookup DE Command– Add to project DE Commands– Might just be a Table (if it’s small)


Creating a Lookup DE Command– Choose Recordset source field (from fetch query)– Specify Lookup Command --> ValidStates table– Specify Display field from Lookup Command– Specify Lookup on Fields column from Lookup

Command


Map Source Command Fields to Lookup Command Fields– Source.State --> Lookup.StateCode

Set GridEditable property to True


Creating a DOW-based UserControl with Lookup column

Part IIIPart III

Implementing ADO Solutions

Implementing ADO Solutions:Implementing ADO Solutions:Effective Queries Effective Queries

Referencing the object ValueReferencing the object Value

Dim Rs as ADODB.Recordset ' ambiguity

...

Rs(0) ' by ordinal

Rs.Fields(0).Value ' explicitly

Rs.Fields(intF).Value ' by variable

Rs(intF) ' by variable

Rs!MyFieldName ' by name

Rs.Fields!Author.Value ' Bang field

Rs("MyFieldName") ' by name

Rs(A$) ' by variable

Speed

Speed

Slowest

Fastest

Referencing Recordset FieldsReferencing Recordset Fields

Referencing Recordset FieldsReferencing Recordset Fields

Use Comments

Use Enumerations

Rs(3) ' Part assembly number

Rs(eRsFld.PartAssy)

Enum eRsFldPartNamePartIDPartAssy

End Enum

Implementing ADO Solutions:Implementing ADO Solutions:Processing Updates Processing Updates

Changing Recordset Data Handling collisions Update strategies Dealing with the errors

Changing Recordset DataChanging Recordset Data

Use Updatable cursor to – Add rows: Use AddNew method

Creates empty row (at end of Recordset)– Change existing rows: Use Update method

No need to use Edit method as in DAO/RDODelete current row: Use Delete method

– Specify delete scope with AffectRecords argument

Use Update method to commit (local) change– Change permanent in “C/S” (non-batch) mode

Use UpdateBatch method to save to DB

Changing Recordset DataChanging Recordset Data

Use SQL action queries to change Recordsets– INSERT statement to add rows – UPDATE statement to change rows– DELETE statement to drop rows

Create WHERE clause using primary key Don’t change primary key

– Use DELETE, INSERT instead

Handling CollisionsHandling Collisions

What can go wrong?– Row changed by another user (or yourself?)– Row deleted– Row “moved”—Primary key changed– Delete fails due to any of the above– Insert fails due to primary key violations– Update or insert fails to field validations– … and many others

3 Copies of your field(s) now available– Was– Is (maybe gone)– Wanna be

Update StrategiesUpdate Strategies

Get, Show Row(s)Get, Show Row(s)Get, Show Row(s)Get, Show Row(s)

Capture ChangesCapture ChangesCapture ChangesCapture Changes

Post to RSPost to RS(EditMode)(EditMode)

Post to RSPost to RS(EditMode)(EditMode)

Stored ProcStored ProcStored ProcStored Proc Client-ManagedClient-ManagedCollisionsCollisions

Client-ManagedClient-ManagedCollisionsCollisions

SQL SQL or Cursor?or Cursor?

SQL SQL or Cursor?or Cursor?

Client-Managed CollisionsClient-Managed Collisions

EnableEnableOn Error GoToOn Error GoTo

EnableEnableOn Error GoToOn Error GoTo

RS.UpdateRS.UpdateRS.UpdateRS.Update

Case (Collision)Case (Collision)Case (Collision)Case (Collision)

Locate changedLocate changedFieldsFields

Locate changedLocate changedFieldsFields

??Force Force throughthrough

AcceptAcceptDBDB

RevertRevert

DoneDone

Using “Unique” PropertiesUsing “Unique” Properties

Unique Table– Specifies base table

Unique Schema– Specifies table owner (or schema)

Unique Catalog– Specifies database

Helps provide specific-table updatability against complex (multi-table) queries

Setting the “Update Criteria” PropertySetting the “Update Criteria” Property

Determines how ADO constructs UpdateWHERE clause– ID + all Columns– ID + Just “touched” columns (changed or not)– ID + Just TIMESTAMP– Just ID (Key)

Client-side cursors only Set before RS.Open in ADO 2.0 Set before or after RS.Open in ADO 2.1

Checking for Update ErrorsChecking for Update Errors

How to tell that the update failed What to do about it Examine the rs.Status property Choosing from the 3 sets of data

– Original data from query (the way it was)– The revised data on your system (the way you

want it)– The revised data on the server (the way someone

else wants it)

Finding what ChangedFinding what Changed

Set the “Update Resync” property (2.1 or later)– adResyncConflicts – retrieves latest values from

DB’s copy of row– Next Update tries to apply these values

OriginalValue = pre-changes from RS CurrentValue = current DB field value

Dealing with the ErrorsDealing with the Errors

Force through your changes—adArrogant

Accept changes already made—adPassive

Revert back to original values—adUncertain

Be prepared for other changes User should not (ideally) be involved

Data Access StrategiesData Access Strategies

Building Success into your apps–Use Stored Procedures–Fewer cursors, fewer rows::more scalability

–Use light-weight cursors–Use GetRows, GetClipString–Move logic out of the client into the middle tier or on to the server

–Avoid VBBLOBs–Avoid API approaches

Visual Basic 6.0 far more “data aware” Makes ambitious strides toward higher

productivity New data tools make development easier

SummarySummary

Questions?Questions?

Documents

Accessing Data Sources with Visual Basic 6.0 William R. Vaughn Microsoft Technical Education (MSTE) Developer Trainer 12