vb .net tutorial - 7

8/14/2019 VB .net tutorial - 7

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Performing Data Updates

NIIT Performing Data Updates/Lesson 7/Slide 1 of 25

Objectives

In this lesson, you will learn to:

Perform cached data updates

Perform direct data updates

Identify the need for concurrency management

Identify the methods of maintaining concurrency in datasets


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Task List

Identify the data that needs to be maintained.

Identify the mechanism to maintain data.

Design a Windows Form to maintain the data.

Connect to the database.

Bind the data to the Windows Form controls.

Write the code to maintain the data.

Perform data maintenance.


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Task 1: Identify the data that needs to be maintained.

Result:

As per the given problem statement, the data to bemaintained is as follows:

CustID

FName

LName

Address

Phone

email


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Task 2: Identify the mechanism to maintain data.

Data can be retrieved from a database directly throughdata commands or cached in datasets.

Performing cached data updates Updating the dataset

While updating the dataset, some events are raised:

ColumnChangingRowChanging

ColumnChanged

RowChanged

RowDeleting

RowDeleted


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Task 2: Identify the mechanism to maintain data.(Contd.)

While updating the dataset, the following information

is maintained:RowState property

Unchanged

Added Deleted

Detached

Modified


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DataRowVersion Enumeration

Original Current

Proposed

DefaultThe changes are committed to the dataset by calling

the AcceptChanges()method.

Updating the data source

The data source is updated with the changeddataset by calling the Update()method of the dataadapter.


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Just a Minute

2. When does the RowState property have the value

Detached?

3. What is the relationship between the RowState propertyand the DataRowVersion enumeration?


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Performing direct data updates

Data commands are can be used to directly update thedata source.

A data command object can be derived from the

OleDbCommand class or the SqlCommand class.

For executing a data command, the Connection,

CommandText, and the Parameters properties for the

data command have to be set.


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Result:

Since you need to access the data through a dataset, youwill perform a cached data update.


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Just a Minute

1. In which cases, direct data update is preferred to updates

through datasets?

2. Write a code to connect to a database through datacommands.


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Task 3: Design a Windows Form to maintain the data.

Task 4: Connect to the database.

Task 5: Bind the data to the Windows Form controls.

Task 6: Write the code to maintain the data.

Task 7: Perform data maintenance.


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Maintaining Data Concurrency

Concurrency control is the process of ensuring that in case

many people attempt to modify data in a database at

the same time, modifications made by one person do not

adversely affect those of another person.

Method Description

Pessimistic concurrency control In this type of concurrency control, a system oflocks applied does not allow users to modify data

in a way that affects other users. When a lock is

applied, other users cannot perform actions that

would conflict with the lock until the owner

releases it. Pessimistic control is mainly used in

environments where the data contention is high

and protecting data with locks is less costly than

rolling back transactions in case of concurrencyconflicts. Pessimistic concurrency is not possible in

a disconnected architecture.


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Maintaining Data Concurrency (Contd.)

Optimistic concurrency control In this type of concurrency control, the lock is not applied when the users read the data. When the

user performs a data update, the system checks to

see if the data has been changed by another user

after the data was read. If the data has been

changed, an error is raised. Optimistic concurrency

is used in environments where the data contention

is low and protecting data with locks is more costly

than rolling back transaction in case of concurrencyconflicts.

Last in Wins concurrency control This type of concurrency control works in a similar

way as the optimistic concurrency control, with the

difference that the record is updated regardless of

whether the record has been changed or not.

Method Description


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Just a Minute

What is the difference between the concurrency control

through the Saving All method and the Version Number

method?


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Optimistic Concurrency through Data Adapter

By default, the data adapters that are created in an

application implement optimistic concurrency.


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Problem Statement 7.P.1

The Sales Manager of Diaz Telecommunications needs to

maintain customer details, which involves adding new

customer details, and modifying and deleting existing

customer details. While adding and modifying details, the

application should ensure that no field is left blank.


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Summary

In this lesson, you learned that:

Data can be retrieved from a database directly through data

commands or can be cached in datasets.

When a dataset is used to access data, updating the

database consists of two steps, updating the dataset and

updating the database.

When you make changes to a record in the table, the

following events are raised by the DataTable object:

ColumnChanging

RowChanging

ColumnChanged


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Summary (Contd.)

The data adapter uses the following information while

updating the data source:

The RowState property The RowState property of theDataRow object indicates the current state of the

record.

TheDataRowVersionenumeration A dataset can

maintain four versions of a DataRow object: Current,Original, Proposed, and Default.

After the changes have been made, they are committed to

the dataset by calling the AcceptChanges() method of the

dataset to accept the changes made to the dataset.


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Summary (Contd.)

The data source is updated with the changed dataset by

calling the Update() method of the OleDbDataAdapter class.

Data commands are generally used in the following cases:

To work with stored procedures that return a result set,

which can be manipulated

To access data that is not appropriate for storing in adataset, such as data with a short life cycle

To access read-only data, that is, data that will not be

updated

A data command object can be derived from theOleDbCommand class or the SqlCommand class.


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Summary (Contd.)

A data command object can be derived from the

OleDbCommand class or the SqlCommand class.

For executing a data command, the Connection andthe CommandText properties for the command

have to be set for specifying the connection and the

SQL query to be executed to retrieve the data,

respectively.

Concurrency means at the same time; thus,

concurrency management means to decide the action

that will take place when multiple users try toupdate the same record.


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Summary (Contd.)

There are three concurrency management methods

available:

Pessimistic concurrency control In this type ofconcurrency control, a record is not available to other

users from the time that a user begins to edit the record

till the time that the record is updated in the database.

Optimistic concurrency control In this type ofconcurrency control, the record is not available to other

users only when the record is being updated in the

database. When a user tries to update a record that has

been changed, an error occurs.


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Summary (Contd.)

Last in Wins concurrency control This type of

concurrency control works similar to the optimistic

concurrency control, the only difference being that the

record is updated without checking whether the recordhas been changed or not.

When the row is changed and there is an attempt to

update the database, ADO.NET uses two methods to

determine if any changes have occurred:

The Version number method

The saving all values method

By default, the data adapters that are created in an

application implement optimistic concurrency.

vb .net tutorial - 7

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