Transcript Tirgul9 - FTP Directory Listing

Enterprise Development
Using Visual Basic 6.0
Autumn 2002
Tirgul #9
‘Tirgul’ # 9
Objectives
• Understanding ADO
• Using DSN/DSN’less connection
• In-depth:
– Connection
– RecordSet
– Command
‘Tirgul’ #9
3-Tier Architecture
‘Tirgul’ #9
The Data Access Layer
• Almost any enterprise application have some
needs to access data (database, web, email,
legacy data..)
• Microsoft has developed a technology to
make this task simple for us
• VB makes it easy to use
‘Tirgul’ #9
The Problem
• Each data source has different way to
access it
SQL Data
•SQL Server
•Oracle
•Informix
•DB2
•Fox Pro
•Jet
Non SQL Data
•Mail
•Video
•Text
•Web
•...
‘Tirgul’ #9
Mainframe and
Legacy Data
The Solution
• Single high level,efficient programming,
paradigm to work with everything it is
called:
universal data access (UDA)
• The programmer can use the same
code to access any data!
‘Tirgul’ #9
What is UDA?
• UDO Consist of:
– ActiveX Data Objects(ADO)
– OLE DB
– And other…
Together these interfaces provide us
the means to work with any data
‘Tirgul’ #9
UDA Architecture
Data consumer
Application (VB, VC++,ASP)
ADO
Data Providers
OLE DB
SQL Data
•SQL Server
•Oracle
•Fox Pro
•Jet
Non SQL Data
0
•Mail
•Video
•Text
•Web
‘Tirgul’ #9
Mainframe
and Legacy
Data
What is ADO?
•
•
ADO is one of the components of the UDA model.
Other components:
– Remote Data Services (RDS)
– OLE DB is Microsoft's component database architecture that provides
universal data integration over an enterprise's network— from mainframe to
desktop - regardless of the data type
– Open Database Connectivity (ODBC) provide a unified way to access
relational data as part of the OLE DB specification
‘Tirgul’ #9
ADO Layer
‘Tirgul’ #9
Ado Object Model Overview
• High level, language independent, data
access interface
• Provides layer to the underplaying data
source (OLE DB Provider)
• Object Oriented interface for accessing data
• ADO is small, lightweight, fast, and feature
complete – everything you need when you
are programming either for the database
applications or the Internet
‘Tirgul’ #9
ADO Objects
• object-based interface that
provides a logical set of objects
you can access from code
Object
Functionality
Connectio
n
Command
Manages the connection with the
data source
Defines the commands that will be
executed against the data source
Contains the data that is retrieved
from the data source
Recordset
‘Tirgul’ #9
ADO Objects
• ADO objects are not dependant on one
another. you can create instances of objects
independently of one another, for example,
you can create a Recordset object without
creating a connection object.
• Unlike the older technologies, ADO is more
flexible, ADO code is easier to write, read and
maintain.
• ADO is built on top of OLE DB and is capable
of accessing any sort of data that is wrapped
and exposed by an appropriate OLE DB
provider.
‘Tirgul’ #9
Basic ADO operations
•
•
•
•
•
•
Select Query
Update Query
Delete Query
Integrity Constraints (PK/FK)
Transactions (Begin, Commit, Rollback)
Stored Procedures
‘Tirgul’ #9
ADO Objects
•
•
•
•
•
•
•
•
Connection
Recordset
Command
Parameter
Error
Field
Property
Stream
‘Tirgul’ #9
The Connection Object
• The connection object enable us to connect to data
stores.
• In this object we specify which:
– OLE DB provider we wish to use.
– Security details for the connection.
– And other specific details.
• NOTE: in some cases you do not have to define a
connection object. Then ADO will create one for you.
as a rule, always define connection object explicitly.
‘Tirgul’ #9
The Command Object
• The command object is designed for
running command against a data store
• You may also execute command using
the connection object but these
command have restrictions.
‘Tirgul’ #9
The Recordset Object
• The most commonly used object.
• It contains a set of data we extract from the data
stores.
• The Recordset holds the record that are usually
fetched from a query
• It allows us to:
– Change the data (update, delete, add).
– Iterate the records.
– Filter the record so that only a subset is shown.
• A Recordset also has a fields collection where there
is a filed object for each column in the Recordset
‘Tirgul’ #9
DISPLAYING DATA WITH A RECORDSET (USING A
CONNECTION OBJECT)
Set Con = new ADODB.Connection
Set RS = new ADODB.Recordset
Con.open “DSN=myDB;UID=sa;Password=;”
RS.ActiceConnection = Con
RS.open “select * from Users”
while not RS.EOF
msgBox RS.fields(“fname”).values
RS.MoveNext
Wend
RS.Close
Con.Close
‘Tirgul’ #9
DISPLAYING DATA WITH A RECORDSET (USING A
CONNECTION OBJECT 2)
StrConnect = “DSN=myDB;UID=sa;Password=;”
Set Con = new ADODB.Connection
Con.open “DSN=myDB;UID=sa;Password=;”
Set RS =new ADODB.Recordset
RS.Open “select * from Users”, Con,
adopenkeyset,adlockoptimistic
while not objRec.EOF
msgBox RS.fields(“fname”).values
RS.MoveNext
Wend
RS.Close
‘Tirgul’ #9
DISPLAYING DATA WITH A RECORDSET (WITHOUT A
CONNECTION OBJECT)
StrConnect = “DSN=myDB;UID=sa;Password=;”
Set RS =new ADODB.Recordset
RS.Open “select * from Users”,strConnect,
adopenkeyset, adlockoptimistic
while not objRec.EOF
msgBox RS.fields(“fname”).values
RS.MoveNext
Wend
RS.Close
‘Tirgul’ #9
ADO 2.5 Extensions
•
•
•
•
Record Object
Stream Object
Not in this course scope
Mainly for handling non relation data
such as File System, Email System
XML Files..
‘Tirgul’ #9
ADO Collections
• There are several collections in the ADO object
library.
– Fields collection.
• This collection is a member of the Recordset
object.
• Each member of the collection is filed object.
• In SQL structured data the field correspond to a
column and contains information about the
column:
– Name, data type, length..
‘Tirgul’ #9
ADO Collections – Cont.
• There several other collections:
– Parameter – part of the command
object
– Errors – part of the connection object
– Properties
‘Tirgul’ #9
Connecting to Data Source
• In order to access some data source, you have to
connect to it using the connection object.
• The actual method of connecting to the data store is
the same for all stores.
• The actual details may be different between different
providers
• There are several ways to connect to data source:
– Using a connection string – DSN’less
– Using ODBC data source – DSN
– Using data link file
‘Tirgul’ #9
DSN
• DSN = 'Data Source Name'
• DSN is an easy way to assign useful
and meaningful names to data sources
which may not be limited to databases
alone (e.g Excel spread sheet etc.)
‘Tirgul’ #9
Creating a DSN (1)
• Database exists
• ODBC32 exists (Check control panel)
– Go into Control Panel
– NT ONLY - Go into Administration
Tools
– Look for ODBC Data Sources (32-bit)
or Data Sources (ODBC)
‘Tirgul’ #9
Creating a DSN (2)
• Go into Control Panel
• Then Administrative Tools
‘Tirgul’ #9
Creating a DSN (3)
• Then ODBC Data Sources (or some
sort of ODBC)
‘Tirgul’ #9
Creating a DSN (4)
• Then click on the System DSN tab and
click on add
User DSN - DSN's for you
only.
System DSN - The whole
system can see these
they are ones that everyone
that accesses your
computer can see.
‘Tirgul’ #9
Creating a DSN (5)
• Choose your driver
‘Tirgul’ #9
Creating a DSN (6)
• Set up the DSN
Enter a Data Source Name, this is what
we call your Database, so keep it short
and without spaces. eg. myBooks
Click on Select... to browse to the
directory where your database is and
choose it, this tells ODBC where you find
the DB (well duh!)
If you set a password for your database
click on Advanced...
‘Tirgul’ #9
Creating a DSN (7)
• Passwords?
Access only supports passwords, not
usernames, so just enter the password
and click OK.
Click OK to the other dialog and you can
see your DSN listed in the System DSN
list, now you can use it to set up
databases.
‘Tirgul’ #9
Create the Northwind DSN
• Northwind is a sample Access DB.
• It comes with Microsoft office
installation.
• Use ‘Search’..
‘Tirgul’ #9
Add Northwind DSN– Select Access Driver
Select Access
Driver
‘Tirgul’ #9
Add Northwind DSN – Select Data base
Use Select to
select database
‘Tirgul’ #9
Add Northwind DSN – Browse for Northwind
‘Tirgul’ #9
Add Northwind DSN – Set name and description
‘Tirgul’ #9
Add Northwind DSN – Done, new in the list
Northwind added
‘Tirgul’ #9
DSN vs DSN less Database Connections
• you can connect to DSN using following
code:
Dim con as new ADODB.Connection
DSN Name only
Other data is
hidden in the DSN
itself
con.Open "DSN=myDSN"
…Use Connection
con.Close
Set con = Nothing
‘Tirgul’ #9
DSN’less Connection
• DSN’less connection does not require
creation of system level DSNs for
connecting to databases and provide an
alternative to DSNs.
• We will now see how to connect to a
database using Connection String in
place of DSN name.
‘Tirgul’ #9
DSN less Connection String
Dim con as new ADODB.Connection
con.Open "Provider=Microsoft.Jet.OLEDB.4.0; Data“ & _
"Source=c:\path\to\database.mdb"
…Use Connection
con.Close
Set con = Nothing
‘Tirgul’ #9
All
Connection
Data
Why to use DSN Connections ?
• Provides meaningful data source
names.
• When there are lots of data sources to
think of and you want a central
repository to hold a collection of data
sources without having to worrying
about the actual configuration of the
data source.
‘Tirgul’ #9
Why to use DSN less Connections ?
• When you can't register DSNs yourself
e.g. when you are running a virtual
hosting account on other's server. Stop
emailing system administrator, connect
to your databases directly.
• Provides faster database access
because it uses native OLE DB
providers which provide faster database
connection.
‘Tirgul’ #9
ADO Connection String
• For MS Access database:
Provider=Microsoft.Jet.OLEDB.4.0;Data Source=c:\somefolder\mydatabase.mdb; User ID=admin; Password=;
• For MS SQL Server:
Provider=SQLOLEDB; Data
Source=server_name_or_address; Initial
Catalog=database_name; User ID=username;
Password=password; Network Library=dbmssocn;
‘Tirgul’ #9
Building a Connection String
• Create a UDL file
– Right click on Desktop,
New -> Text File,
rename to filename.UDL
• Double-click the UDL file
• Select Microsoft.Jet.OLEDB.4.0
‘Tirgul’ #9
Building a Connection String (cont.)
• Click Next >>
• Browse for the Access
database
• Enter login information
if necessary
‘Tirgul’ #9
Building a Connection String (cont.)
• Click OK
• Open the UDL file using Notepad
‘Tirgul’ #9
Connection – Open Method
• The connection object has an open method:
– Connection.Open [connetionstring],
[user id], [password], [options]
ConnectionString
The String containing the connection details
UserID
The name of the user during the connection.
Overides the user name in the connection
string
Password
Same as user but password
Options
adAsyncConnect or empty
‘Tirgul’ #9
Establishing a SQL Database
Connection
Set a reference to the
ADO object library
To set a reference to the ADO Library in VB, click
the Project menu, and click Reference. Then
select the appropriate reference
Declare a Connection
Object
Dim conn as ADODB.Connection
Set conn = new ADODB.Connection
Specify an OLE DB
Data Provider
Conn.Provider = “SQLOLEDB”
Specify Connection
Information
Use the connection string property to specify any
additional information
strCnn = "Data Source=ds;uid=sa;pwd=;” & _
database=pubs;Initial Catalog=pubs"
Open a connection
Use the open method to establish connection to
the data source
conn.open strCnn
‘Tirgul’ #9
Opening a Connection
‘Tirgul’ #9
Closing a Connection
• When you finished working with the data source, use close
method of the connection object.
– Free any associated system resources
– Close any active Recordset associates with this connection
– Set the object to nothing in order to move the object from
memory
Conn.Close
Set Conn = nothing
– NOTE: connection will be closed automatically when the
connection variable goes out of scope or set to nothing
– Closing connection is very important for scalability and
performance especially when it is being used over the
web (connection pooling)
‘Tirgul’ #9
The Connection Object - Interface
• Properties
• Methods
– CommandTimeout
– BeginTrans
– ConnectionString
– Close
– ConnectionTimeout
– CommitTrans
– CursorLocation
– Execute
– DefaultDatabase
– Open
– IsolationLevel
– OpenSchema
– Mode
– RollbackTrans
– Provider
– State
– Version
• Collections
– Errors
– Properties
‘Tirgul’ #9
Recordsets
• Recordset contains the data as a table.
• Knowing the details of how the data is
held is and manipulated is essential.
• It allows you to make informed decision
of what type of Recordset you are going
to use.
‘Tirgul’ #9
Recordset & Cursors
• Like every other object a Recordset has data,
methods and state.
• Each Recordset set has one pointer to the current
record.
• The curser is what manage the set of records and
the current location within the record
• When you iterate through records, the cursor
performs the move. When you try to move beyond
the last record the curser handles it.
‘Tirgul’ #9
Recordset & Cursors
Current Record
ID
111111
222222
333333
Name
Bob
Mary
Bil
Cursor Engine
‘Tirgul’ #9
Phone
555555
666666
777777
Cursors Type
• Static (adOpenStatic).
– Contains a static copy of the record
– The content of the Recordset are at the time the
record is being created.
– Any record modified, added or deleted by other
user will not be visible.
– Movement through the Recordset is allowed both
forward and backward
• Forward only (adOpenForward)
– The default cursor type
– Identical to static except you can only move
forward through the Recordset
‘Tirgul’ #9
Cursor Types – Cont.
• Dynamic (adOpenDynamic).
– Doesn’t have fixed set of records
– Any changes by other users will be visible in the
Recordset
– Movement is allowed in both directions
• Keyset (adOpenKeyset).
– Like dynamics, any changes are visible except
new records.
– If other user delete a record this record will be
inaccessible by the Recordset
‘Tirgul’ #9
Performance issue
• Why are static cursor are slower the forward
–only cursor?
– Forward cursor do no have to keep any
track about records they past, static do!
• If you just want to look at the records in order
to build a table, what kind of cursor would you
be using?
– Forward only
‘Tirgul’ #9
Recordset and Locks
• Before getting into recordset creation and
manipulation. There is another last topic – Locks
• Locking is a mechanism to ensure Integrity of the
data, making sure that changes aren’t overwritten
• We want to make sure that my changes of the data
will be overwritten by other user changes
• To manage this protection we use locking.
• There are several type of recordset locks
‘Tirgul’ #9
Lock types
• Lock Types:
– Read Only (adLockReadOnly) – The default
locking type. Recordset is read only.
– Pessimistic (adLockPessimistic) – Locking a
record whenever editing is taking place
– Optimistic (adLockOptmistic) – The record is
not locked until changes to the data are committed
to the data store
– Batch Optimistic (adLockBatchOptimistic).
Allows multiple records to be modified, and the
records are only locked when the UpdateBatch
method is called
‘Tirgul’ #9
Performance issue
• If you do not need to modify records
what locking would you be using?
– Read-only
• Pessimistic insures more integrity, why
not use it always?
– Concurrency, a locked records is
available to other user therefore
concurrency is reduced
‘Tirgul’ #9
Creating a Recordset
• Creating Recordset is achieved by using the open
method
•
Recordset.Open [source] , [ActiveConnection] , [CursorType] ,
[LockType], [options]
Source
The source of the data. Can be the name of the table is the
database, an SQL string, a stored procuder or a command
object
ActiveConnection
The connection to use for this connection. Can be an open
Connection object or a connection string
CursorType
The type of cursor to use. The default is adForwardOnly
LockType
The type of locking to use. The default is adLockReadOnly
Options
Tells the provider what the source argument is – that is,
whether is is a table, a text string and so on
‘Tirgul’ #9
Creating Recordset
An opened
connection object
‘Tirgul’ #9
The Recordset Object
•
Properties
•
AbsolutePage, AbsolutePosition,
ActiveConnection, BOF,
Bookmark, CacheSize,
CursorLocation, CursorType,
EditMode, EOF, Filter, LockType,
MarshalOptions, MaxRecords,
PageCount, PageSize,
RecordCount, Source, State,
Status
•
• Collections
– Fields, Properties
‘Tirgul’ #9
Methods
– AddNew, CancelBatch,
CancelUpdate, Clone,
Close, Delete, GetRows,
Move, MoveFirst,
MoveLast, MoveNext,
MovePrevious,
NextRecordset, Open,
Requery, Resync,
Supports, Update,
UpdateBatch
Navigating Through Records
• Once you opened a record you often need to loop
through each record
rsAuthors.Open “authors” , conn
Do While Not rsAuthors.EOF
Debug.Print rsAuthors(“au_lname”) & “,” & _
rsAuthors(“au_fname”)
rsAuthors.MoveNext
Loop
EOF property is
true when the
end of the
Recordset has
been reached
• If your Recordset allow moving backwards you can
use MovePrevious and check the BOF property
• There are also MoveFirst , MoveLast and Move
methods
‘Tirgul’ #9
Using the Fields Collection
• The fields collection contains a field for
each column in the Recordset.
• The fields collection is the default
collection and therefore can be omitted
when accessing fields.
• The following example all return the
same result.
‘Tirgul’ #9
Using the Fields Collection
rsAuthors.Fileds(“au_lname”).Value
rsAuthors(“au_lname).Value
Default collection
rsAuthors(1).Value
rsAuthors.Fields(1).Value
rsAuthors(“ay_lname”)
Default collection &
Property
rsAuthors(1)
Don’t forget to use Trim when add/Select a String
‘Tirgul’ #9
Using the Fields Collection
To Get Meta Data
‘Tirgul’ #9
Bookmarks
• When you are moving around the Recordset you might want to
retain the position of a record and then move back to it later
• A Recordset move bookmark holds a unique pointer to an
individual record
• To Bookmark you simple assign the bookmark property to a
variable
Dim varBkMark as Variant
varBkMark = rsAuthors.BookMark
• To return to the bookmark position you do the opposite
rsAuthors.BookMark = varBkMark
NOTE: Not all records support Bookmarks. The support method allows you to identify that
If rsAuthors.Support(adBookmark) then…
‘Tirgul’ #9
Filtering Recordsets
• Filtering is a way to temporarily restricting the
view of records in a Recordset
• It is useful if you want to show subset of
records without re-queering the data store
rsAuthors.Filter
rsAuthors.Filter
rsAuthors.Filter
rsAuthors.Filter
=
=
=
=
“State=‘ca’”
“au_lname = ‘homer’ or au_lname = ‘Francis’”
“au_lname LIKE ‘Ho%’”
“” ‘Reset filtering
‘Tirgul’ #9
Filtering With Constants
• The filter property can also take one of the
FilterGroupEnum constants as its
argument. These are:
– adFilterNone – removes current filter
– adFilterpendingRecords – to view
only records that have changed but not yet
been sent to server (batch update move)
• Look in the MSDN for the other constants
‘Tirgul’ #9
Filtering with Bookmarks
The filter is applied
on the bookmarks
array
The Array function
must be used to
convert the individual
bookmark to a variant
array
‘Tirgul’ #9
Searching for Record
•
Searching for individual records is performed using the find method
rsAuthors.Find = “au_lname = ‘loyd’”
•
•
You can only have one criterion – And or Or is not allowed
You can use optional argument to specify extra options
Recordset.Find = Criteria , [SkipRow], [SearchDirection],[Start]
SkipRow
Number of rows to skip before search (0 by default)
SearchDirection
Can be either adSearchForward or adSearchBackWard
Start
Start is a Bookmark identifing where to start from
•
•
If record is found you are placed at that record
Else you are either at EOF or BOF depending on the search direction
‘Tirgul’ #9
Searching for Record – Using Bookmark
‘ Save current position
varBkmk = rsAuthors.BookMark
‘Find the Record
rsAuthors.Find “au_lname = ‘Sussman’”
‘Was it found
If Not rsAuthors.EOF then
Debug.write “Found: “ & rsAuthors(“au_lname”)
Else
Debug.write “Not Found. Moving”
rsAuthors.Bookmark = varBkmk
End If
‘Tirgul’ #9
Modifying Records
• Up to now we saw methods and properties to query
the Recordset.
• However many times we want to modify records:
– Using: Add, delete, update..
• Modifying records can be done both using the
Connection, Command and Recordset objects.
• Each method has advantages and disadvantages.
To modify a record in recordset you must have locking
type other than AdLockReadOnly – remember
this is the default.
‘Tirgul’ #9
Adding Records using Recordset
• There are two ways to add records, both use the AddNew
method:
With rsAuthors
.Open “authors”, conn, adOpenDynamic, _
adLockOpimistic, adCmdTableDirect
.AddNew
.Fields(“ui_id”) = “123-12-1234”
.Fields(“au_lname”) = “Lloyd”
.Fields(“au_fname”) = “Janine”
.Fields(“Contract”) = 0
.Update
End with
Requires the Update
method
With rsAuthors
.Open “authors”, conn, adOpenDynamic, _
adLockOpimistic, adCmdTableDirect
.AddNew Array(“ui_id” , “au_lname” , “au_fname” , “Contract”), _
Array(“123-12-1234”, “Lloyd”, “Janine”, 0)
End With
‘Tirgul’ #9
Editing Records using Recordset
• Editing records is similar to the first method of inserting recordsthe difference being that you don’t need to call the AddNew
method
strSQL = “SELECT * FROM authors WHERE au_lname = ‘Lloyd’”
With rsAuthors
.Open strSQL , conn, adOpenDynamic, _
adLockOpimistic, adCmdTableDirect
.Fields(“Contract”) = 1
.Update
End with
The example above simply set the “contract” field of the
current record (in this case the first record) to 1
‘Tirgul’ #9
Deleting Records using Recordset
• To Delete records you call the Delete method. You
may add optional argument from the Affect Enum
constants
– adAffectCurrent – delete the current record
(default)
– adAffectGroup – all records matching the
current filter will be deleted
– adAffectAll – all records in the recordset will
be deleted
rsAuthors.Delete
Deletes the current record
‘Tirgul’ #9
Operations using Connection
• All operations (Select/Add/Edit/Delete) could
be executed using the Connection object:
• For Select (Returned Recordset)
Dim Conn As New ADODB.Connection
Dim RS As ADODB.Recordset
Set RS = Conn.Execute(SQLCommand, RecordsAffected)
SQL command
• For Add/Edit/Delete(No need for Recordset)
Dim Conn As New ADODB.Connection
Conn.Execute SQLCommand, RecordsAffected
‘Tirgul’ #9
Operations using Commend
• All operations (Select/Add/Edit/Delete) could be
executed using the Command object.
• For a row-returning Command:
Dim Command As New ADODB.Command
Dim RS as new ADODB.RecordSet
Set RS = Command.Execute( RecordsAffected, Parameters, Options )
• For a non–row-returning Command:
Dim Command As New ADODB.Command
Command.Execute RecordsAffected, Parameters, Options
‘Tirgul’ #9
Working with Command
• Assume we have an open connection:
Dim Command As New ADODB.Command
Set Command.ActiveConnection = Conn
Command.CommandText = strSQLCommand
Command.Execute
‘Tirgul’ #9
Working with Command parameter
• Common for stored procedures
Dim adoCn As ADODB.Connection
Dim adoCmd As ADODB.Command
Dim adoParam As ADODB.Parameter
Set adoCmd = New ADODB.Command
With adoCmd
.CommandType = adCmdStoredProc
.CommandText = “SP_Cart_DeleteAll"
'***Build parameters collection
Set adoParam = .CreateParameter(Name:="RETURN_VALUE",
Type:=adInteger,Direction:=adParamReturnValue)
.Parameters.Append adoParam
Set adoParam = .CreateParameter(Name:="@SessionID",
Type:=adInteger, Size:=4,Direction:=adParamInput,
Value:=SessionID)
.Parameters.Append adoParam
End With
adoCmd.ActiveConnection = GetConnection()
adoCmd.Execute
‘Tirgul’ #9
Working with Command parameter
Public Const GET_STUDENT_DATA_QUERY = “Select
* from Students where StudentID = ?”
Dynamic variable
Dim Command As New ADODB.Command
Dim param as ADODB.Parameter
Set Command.ActiveConnection = Conn
Command.CommandText = GET_STUDENT_DATA_QUERY
Command.CommandType = adCmdText
Set param = MyCommand.CreateParameter(, adVarChar,
adParamInputOutput, 50)
param.value = “1”
Command.Parameters.Append param
Command.Execute
‘Tirgul’ #9
Auto Increment Field Problem
•
•
Assume you have a table “tblTest” containing two files
– ID – auto incremented integer
– Name – text
What will the following code produce?
With rsData
.Open “tblTest” , conn, adOpenDynamic, _
adLockOpimistic, adCmdTableDirect
.AddNew
.Fields(“Name”) = “Janine”
.Update
Debug.Print .Fields(“ID”)
End with
•
Seems trivial but whether you can obtain this value after adding a new
record depends upon the cursor type, lock type, and whether ID is
indexed or not.
‘Tirgul’ #9
Auto Increment Field Problem
•
Any of the following combinations will return the new value
Provider
Target
Indexed
Cursor
Location
Cursor
Type
Lock
Type
Jet 4.0
Access97
Yes
Server
All
All
Access2000
Yes
Server
All
All
No
Client
All
Pessimistic
Optimistic
Yes
Server
keySet
Pessimistic
Optimistic
Client
All
Pessimistic
Optimistic
Client
All
Pessimistic
Optimistic
SQL OLE
DB
SQL Server 7.0
No
‘Tirgul’ #9
Managing Errors
• When dealing with data store many run
time errors may arise :
– Security problems.
– Synchronization – updating deleted
record.
– Connection – network..
• You can’t guarantee that every think is
going to work perfectly.
‘Tirgul’ #9
The Errors Collection
• The errors collection contains an error object for each
error that accurred during a single ADO command.
• The OLE DB provider set this objects.
• Two important points to remember.
– Each time an ADO command is executed, if an
error occurs , the collection is cleared and filled. If
no error occurs than the collection is untouched.
– The OLE DB provider may fill the error collection
with information and warning messages. These
always have error number 0.
‘Tirgul’ #9
The Errors Collection
• The collection is part of the connection
object
– Connection.Errors
• You can also access the collection from
the Recordset object
– rsAuthors.ActiveConnection.Errors
For Each errAuthors in rsAuthors.ActiveConnection.Errors
‘Handle/Display Error
Next
‘Tirgul’ #9
The Error Object
• Each member of the errors collection is
an error type
Property
Description
Number
The ADO Error number
NativeError
The error number from the data provider
SQLState
The 5 digit SQL state code for the error, when connecting to
an SQL database
Source
The Object that generated the error
Description
Descriptive text of the error
‘Tirgul’ #9