Hướng dẫn học Microsoft SQL Server 2008 part 77 pptx

The structure is consistent and simple, always with the following progression: Parent Object Collection of dependent objects Child Object OLEDB data providers Even when ADO is used thro

Open Database Connectivity (ODBC) is a technology for connecting applications to databases that has been in use longer than OLEDB Unlike OLEDB, ODBC is designed for connecting only to RDBMS sources The ODBC driver is included in the SNAC, however.

A small set of objects is used to work with ADO Table 32-1 lists these objects and describes how to

use them Most of these object types have a counterpart in predecessor technologies that Microsoft has

introduced, although the level of ADO-object functionality is much greater than that offered by

previ-ous technologies and, as demonstrated next, the potential usability for more recent technologies such as

ADO.NET and XML transcends even ADO

TABLE 32-1

ADO-Object Overview Object Description

Connection A connection object defines the connection with the OLEDB provider Use this

object to perform tasks such as beginning, committing, and rolling back transactions

There are also methods for opening or closing the connection and for executing commands

Error ADO creates an error object as part of the connection object The error object

provides additional information about errors raised by the OLEDB provider A single errorobject can contain information about more than one error Each object is associated with a specific event, such as committing a transaction

Command A command object performs a task using a connection or recordset object Even

though commands can be executed as part of the connection or recordset object, the command object is much more flexible and enables the definition output parameters

Parameter The parameter object defines a single parameter for a command A parameter

modifies the result of a stored procedure or query Parameter objects can provide input, output, or both

Recordset The recordset object contains the result of a query, and a cursor for choosing

individual elements within the returned table

Record A record is a single row of data It can stand alone or be derived from a record set

Field A field object contains a single column of data contained in a record or recordset

object In other words, a field can be thought of as a single column in a table; it contains one type of data for all the records associated with a record set

Stream When a data provider is not able to easily express the value and length of the data as

a record set with discrete fields, as is the case for large text, BLOB, or document data, the data may be sent to the consumer via the stream object

Property Some OLEDB providers will need to extend the standard ADO object Property

objects represent one way to do this A property object contains attribute, name, type, and value information

There are also four object collections in ADO: Errors, Parameters, Fields, and Properties Note that these

collections are containers for child objects in the ADO model There are no collections at the root of the

object model, and the model is never more than two levels deep The structure is consistent and simple,

always with the following progression: Parent Object Collection of dependent objects  Child Object

OLEDB data providers

Even when ADO is used through the provided NET primary interop assembly, all data access will

occur through one of the available OLEDB COM data providers A data provider manages the connection

between the client and the DBMS using a number of objects Of course, this means that a data

provider requires a source of information and must define the specifics for creating that connection

Generally, a provider is database specific or provides a means for configuring a specific database

Figure 32-3 shows a typical list of database providers Some of the providers on the list are quite

specialized

FIGURE 32-3

A typical list of database providers

The source of an OLEDB object is known as a provider Consequently, ADO also relies on data

providers as a source of data Even in this day of the NET Framework, the number of OLEDB

providers — each specialized to a particular data source — is greater than at any time in the past

One nice thing about OLEDB is that the same provider works with any Visual Studio programming

language

Generally, especially for SQL Server developers, it’s better to use the SQL Server–specific provider Even

though other general-purpose providers will work, Microsoft has optimized the SQL Server provider

for use with SQL Server The performance advantages of using the SQL Server provider over using a

general-purpose provider are well known

To find out which OLEDB providers are available on a particular machine, create a new text file and rename it to include an extension of ‘‘ udl ’’ (e.g., temp.udl — the name is not important, only the extension) Open the file to see a dialog similar to the one shown in Figure 32-3

showing all the OLEDB providers installed locally on that computer.

Mapping data types

When working exclusively within SQL Server, the challenge with data types amounts to choosing the

right type for a given data-storage need However, when moving the data from the DBMS through a

data provider to a client, several layers of transition occur For some DBMSs this is an extreme problem

because the general providers supplied with OLEDB don’t support many special data types For all

providers, the cost of data type conversions as data propagates through those layers is significant These

problems of data typing are additional reasons to use the SQL Server–specific OLEDB data providers

when working with ADO

When using data found in a SQL Server table in a client application, the provider must map the data

from a type that SQL Server understands to a type that the client application understands Fortunately

for SQL Server developers, the ADO mapping for the SQLOLEDB provider is relatively straightforward

Table 32-2 shows how the SQL Server provider maps ADO data types One problem occurs when

ADO uses the same data type to represent two or three SQL Server data types when the application

requires subtle differences to appear in the user interface The complete set of SQL Server 2008 data

types is exposed in the SQL Native Client ADO is only aware of the SQL Server 2000 data types and

will not be able to handle the new types, such asXMLor avarchar(max), discussed later in this

chapter

Table 32-2 shows the SQL Server 2008 data types and the equivalent ADO data types, along with the

data type conversion that the NET Framework will conduct for each ADO data type

The comments in Table 32-2 touch on the most significant problems that can occur in the mapping of

data between ADO and SQL Server It is important to also consider data-conversion errors According

to Microsoft, all nondirect data translations are subject to data loss For example, neither the provider

nor SQL Server will complain if an eight-byte number is converted into a four-byte number, but

obvi-ously there is a potential for data loss In addition, some types cannot be converted to other types For

example, it’s impossible to convert anadBinarydata type into anadSmallIntdata type In this

sit-uation, the development environment would complain The sort order of SQL character data types is, by

default, different from the sort order of NET data types

The NET Framework adds another level of conversion and potential conversion errors to the ADO

implementation ADO data types are specified within ADO objects The NET Framework will convert

those data types to valid NET data types implicitly when they are used outside of the ADO objects in

the code This conversion should happen without the need for explicit data conversion type casting,

although it is good programming practice to ensure that data is always of the type expected The

developer must remain vigilant for data type conversion problems when programming ADO, even in the

.NET Framework Using the NET data types that support null values may provide relief for conversion

problems with many applications There seems to be no perfect solution to this problem short of

environment coding standards that mandate consistent best practice programming techniques for all

programmers in the environments, and rigorous testing

Best Practice

To avoid unexpected data type conversions at assignment, ADO data types can be assigned using explicit

casting to the desired data type

TABLE 32-2

SQL Server to ADO/ADO.NET Data Mapping ADO Data Type

SQL Server (.NET Framework

Data Type Data Type) Notes

Bigint adBigInt

(int64)

The bigint data-type value ranges from -2 ˆ 63 (-9,223,372,036,854,775,807) through 2 ˆ 63-1 (9,223,372,036,854,775,807) This value is only available for SQL Server 2000, but the OLEDB provider will still try to send it to SQL Server 7.0 and older systems, and data loss will result Use the adBigInt type only when necessary and then with caution

Binary adBinary

(byte[])

ADO uses the same data-type equivalence for both binaryand timestamp

(Int16)

While this data transfer always works, conceptual differences exist between the two For example, a bit can have values of 1, 0, or NULL, whereas an adBooleanalways has either a true or false value

(string)

ADO uses the same data-type equivalence for char, varchar, and text data types The NET Framework uses Unicode (UTF-16) to represent all character data

Date 0001-01-01 through 9999-12-31 Three bytes storage; is

not time zone offset aware or daylight saving time aware

Datetime adDBTimeStamp

(DateTime)

The default precision for the Datetime data type in SQL Server is 3.33 milliseconds

Datetime2 0001-01-01 through 9999-12-31 Six bytes storage for

precision less than 3, 7 bytes for precisions 4 and 5, 8 bytes for all other precisions

Datetimeoffset 0001-01-01 through 9999-12-31 Ten bytes storage

Decimal adNumeric

(Decimal)

ADO uses the same data-type equivalence for both decimaland numeric data types

continued

TABLE 32-2 (continued )

ADO Data Type SQL Server (.NET Framework

Data Type Data Type) Notes

Float adDouble

(Double) Hierarchyid

Image adVarbinary

(byte[])

This data type can be so large that it won’t fit in memory The lack of memory can cause provider errors and possibly only a partial retrieval When this happens, the developer must write a custom routine to retrieve the data in pieces ADO uses the same data-type equivalence for image, tinyint, and varbinary

(Int32)

−2,147,483648 to 2,147,483647 Storage of 4 bytes

Money adCurrency

(Decimal)

ADO uses the same data-type equivalence for money and smallmoney

Nchar adWChar

(string)

ADO uses the same data-type equivalence for nchar, ntext, nvarchar, and sysname The NET Framework uses Unicode (UTF-16) to represent all character data

Ntext adWChar

(string)

This data type can be so large that it won’t fit in memory The lack of memory can cause provider errors and possibly a partial retrieval When this happens, the developer must write a custom routine to retrieve the data in pieces ADO uses the same data-type equivalence for nchar, ntext, nvarchar, and sysname The NET Framework uses Unicode (UTF-16)

to represent all character data

Numeric adNumeric

(decimal)

ADO uses the same data-type equivalence for both decimaland numeric data types

Nvarchar adWChar

(string)

ADO uses the same data-type equivalence for nchar, ntext, nvarchar, and sysname The NET Framework uses Unicode (UTF-16) to represent all character data

NvarChar(MAX) None (string) SQL Server 2008 provides the same data-type

equivalence for what was previously Nvarchar if less than or equal to 8 KB, and text if greater than 8 KB

The NET Framework uses Unicode (UTF-16) to represent all character data

continued

TABLE 32-2 (continued )

ADO Data Type SQL Server (.NET Framework

Data Type Data Type) Notes

(Single) Smalldatetime adTimeStamp

(DateTime) Smallint adSmallInt

(Int16)

−32,768 to 32,767 Storage of 2 bytes

Smallmoney adCurrency

(Decimal)

ADO uses the same data-type equivalence for money and smallmoney

sql_variant adVariant

(object)

This data type can contain any of a number of primitive data types, such as smallint, float, and char It can’t contain larger data types such as text, ntext, and image The adVariant type maps to the OLEDB DBTYPE_VARIANT data type and is only usable with SQL Server 2000 Be careful when using this data type because it can produce unpredictable results Even though OLEDB provides complete support for it, ADO doesn’t

Sysname adWChar

(string)

ADO uses the same data-type equivalence for nchar, ntext, nvarchar, and sysname The NET Framework uses Unicode (UTF-16) to represent all character data

Table

(string)

This data type can be so large that it won’t fit in memory The lack of memory can cause provider errors and possibly a partial retrieval When this happens, the developer must write a custom routine to retrieve the data in pieces ADO uses the same data-type equivalence for char, varchar, and text data types

The NET Framework uses Unicode (UTF-16) to represent all character data

Time An integer from 0 to 7 Specifies the fractional part of

the seconds 00:00:00.0000000 to 12:59:59.9999999

Five bytes storage

Timestamp adBinary

(byte[])

ADO uses the same data-type equivalence for both binaryand timestamp

continued

TABLE 32-2 (continued )

ADO Data Type SQL Server (.NET Framework

Data Type Data Type) Notes

Tinyint adTinyInt

(byte)

0 to 255 Storage of 1 byte

Uniqueidentifier adGUID (Guid) The data provider supports a string GUID, not a true

GUID This means that when an actual GUID is needed, the code must explicitly convert it into a GUID data structure

Varbinary adVarbinary

(byte[])

ADO uses the same data-type equivalence for image and varbinary

Varbinary(MAX) none (byte[]) SQL Server 2008 provides the same data-type

equivalence for what was previously varbinary if less than or equal to 8 KB, and image if greater than 8 KB

Varchar adChar

(string)

ADO uses the same data-type equivalence for char, varchar, and text The NET Framework uses Unicode (UTF-16) to represent all character data

Varchar(MAX) None (string) SQL Server 2008 provides the same data-type

equivalence for what was previously varchar if less than or equal to 8 KB, and text if greater than 8 KB

The NET Framework uses Unicode (UTF-16) to represent all character data

ADO and scripting

ADO often appears in scripts of various types Because ADO relies on COM technology, any scripting

language capable of creating an object can probably use ADO to retrieve data from a database Using

scripts to perform small tasks makes sense because they can easily be modified if necessary and they are

quick to write Be aware that scripting facilities such as the Windows Script Component can be created

and referenced from VB.NET or C#, and the Windows Scripting Host runs ADO as a COM component

only Interoperability with NET does not apply

.NET code is compiled to IL Because the compilation to IL must happen at some point before runtime,

the concept of late binding is not available to NET code Conversely, script-based ADO, such as that

used in the Windows Scripting Host 5.6, a SQL Server 2000 DTS ActiveX script step, or a pre NET

ASP page, requires late binding Late binding simply means that the COM object referenced is created

at runtime and is identifiable by the use of theCreateObject()function Therefore, to use ADO

in scripts, MDAC must be installed on any machine that will execute scripts In contrast, ADO.NET

will be fully supported by the SQL Native Client and will not require MDAC This important new

distinction with ADO.NET 2.0 (and subsequently ADO.NET 3.5) is covered in greater detail later in

this chapter

Of course, scripting languages don’t provide the robust interactive environment found in programming

languages such as C# or Visual Basic or even ASP.NET It may be necessary to restrict the use of scripts

to small tasks such as calling on a stored procedure to perform some task automatically or to retrieve

the result of a data query for onscreen display

Microsoft makes a point of demonstrating the flexibility of ADO with several languages, including Java,

JavaScript, VBScript, and the new XML-based Windows Script Component available in Visual Studio

2005 as well as 2008

ADO.NET

It’s somewhat confusing that ADO.NET inherits the ADO part of its name from the original ADO ADO

is the acronym for ActiveX Data Objects ActiveX is a clear signal that the topic isIUnknownand COM

IUnknownis the prototype for all COM classes In its raw form it indicates that the calling module

needs to know nothing about a called object and the caller will still be able to statefully interact with

the called module While deep down in the bowels of NET there still exists COM programmability, the

whole point of the NET Framework and the common language runtime (CLR) is the shift away from

the limitations of the COM code execution environment and toward consistency in the object-oriented

class model.IUnknownmeans there is no need to know anything about a class to instantiate and

use the class The NET Framework signifies that the system assemblies and the base classes will be

reliably consistent types In short, there is little that is COM-based ADO in ADO.NET from a technical

perspective

Only from a functional perspective is ADO.NET a natural progression from ADO A new technology

was required to overcome the limitations of ADO performance and scalability — a technology that

provided the developer with a variety of execution options ADO.NET was designed to both overcome

ADO limitations and leverage the developer’s ADO skills The basic objects of ADO are found in

ADO.NET Commands executed on connections to data providers are used in ADO.NET with only slight

differences from the commands executed on connections to data providers used with ADO However,

with ADO.NET, developers have greater control over how the data will be retrieved and manipulated

Execution can be asynchronous and batched Results can be elegantly stored and manipulated at the

application, disconnected from the database Alternately, results can as easily be streamed as binary data

or as XML

Many developers labor under the misconception that ADO.NET is simply the upgrade to ADO ADO

was developed to support client/server applications and presupposes that the user and the data will both

remain connected to the application for the lifetime of an execution cycle At the risk of oversimplifying

the difference, ADO held state in the data source and ADO.NET is built to be able to maintain state

dis-connected from the database One advantage for NET is that there is no requirement that the database

remain connected to the application for the complete execution cycle In part, this design goal is

real-ized because XML technology is fundamental to ADO.NET To a larger degree, state is managed in the

ADO.NET application layer by the local application cache known as the DataSetclass In ADO.NET

2.0 and later, state management is further extended with asynchronous command execution and Multiple

Active Result Sets (MARS).

The following sections describe ADO.NET objects, keeping ADO objects in perspective, and point

out the new features in the ADO.NET 3.5 release In other words, the discussion builds on the ADO

information presented earlier in this chapter as ADO.NET concepts are introduced ADO.NET is a

man-aged object model with functional capabilities much like the classic ADO, yet designed to enable more

scalable applications, particularly in the disconnected environments found in many n-tier, SOA, Web

Services, and ASP.NET applications

The ADO.NET object model

The ADO.NET object model is different from the object model used by ADO It is more complex yet

undeniably richer The most striking difference is the in-memory data cache known as theDataSet

This ADO.NET object can be divided into two components: theDataSetand the data provider A data

provider is used for connecting the application to a database, executing commands and returning the

results to the application The results that are returned to the application are either processed directly

or placed into a DataSet

ADO.NET provides many data providers in order to access the many types of data sources such as

Microsoft SQL Server, Access, Excel, and others These and other data providers are very lightweight

and create a very thin layer between application source code and the data source This provides

great performance without giving up functionality The data provider contains the classes that create

the connection, issue commands, handle the data reader, and provide data-adapter support The

connectionprovides the conduit for database communications Thecommandenables the client to

request information from the database server through thedata adapter In addition to providing data,

the data adapter also enables the client cache to synchronize or update back to the data source The

data readeris a one-way, read-only method of viewing data in ADO Thedata adapterprovides

the real-time connection support normally associated with live data connections

TheDataSetis a special object that contains one or more tables The data in theDataSetis retrieved

from the data source through the provider and stored in the application work space The data can be

manipulated and constrained at the application TheDataSetis a disconnected subset of the data

from the data source defined by the provider properties The data provider properties include the

Connection,Command,DataReader, andDataAdapterobjects Each of these objects also has

capabilities not found in an ADO provider For example, aDataAdaptercan actually handle more

than one connection and one set of rules

As with many managed objects, enumerators are used to access the various objects within these main

objects in application code The DataSet is the representation of information within the database

It contains two collections:DataTableCollectionandDataRelationCollection The

DataTableCollectioncontains the columns and rows of the table, along with any constraints

imposed on that information TheDataRelationCollectioncontains the relational information

used to create theDataSet

Table 32-3 provides an overview of the most frequently used ADO.NET data classes

Visual Studio 2008 provides a TableAdapter , which is a single-table data collection that can be used in application code much like an ADO.NET 3.5 DataSet The TableAdapter

is significantly easier for the developer to create and manipulate than the underlying ADO.NET

3.5 data components when only one table is needed at the application The TableAdapter is not

derived from the System.Data.DataSet class, as are all other DataSets In fact, TableAdapters

are not even part of the NET Framework They are a level of abstraction provided by Visual Studio

2008 All TableAdapters inherit from System.Component.ComponentModel This means that they

are fully integrated with the Visual Studio tools, such as the Data Grid In addition, the base class

for a TableAdapter can be any of the NET providers that expose a DataTable class The base

class is specified when the TableAdapter is created TableAdapters are type-safe and include all

properties and methods necessary to connect to a data source, retrieve the table’s data, and update

the data source You can find more information about the TableAdapter on the MSDN website:

http://msdn.microsoft.com/en-us/library/7zt3ycf2.aspx

TABLE 32-3

ADO.NET 3.5 Class Overview Class Type Description

Connection Creates the physical connection between the DBMS and a DataAdapter,

DataReader, or factory The Connection object also includes logic that optimizes the use of connections within the distributed application

environment

ProviderFactory Implemented in ADO.NET 2.0 Each NET provider implements a

ProviderFactoryclass, each of which derives from the common base class DBProviderFactory The factory class includes methods for creation of provider-specific ADO.NET components in a generic code style The idea behind the ProviderFactory is to enable the developer to write generic code that can use a provider determined at runtime The possible providers a factory can use are stored in the machine.config file

Command Defines an action to perform on the DBMS, such as adding, deleting, or

updating a record The DataAdapter includes the command objects required

to query, delete, insert, and edit records

Parameter A parameter to a command

Error The error or warning information returned from the database For SQL Server

this includes the error number, the severity, and the text for the error

Exception The application exception when ADO.NET encounters an error The Error

class is created by the Exception class The Exception class is used in ADO.NET try-catch error handling

DataAdapter Translates the data from the data provider source into the in-memory DataSet

or DataReader The DataAdapter performs all queries, translates data from one format to another, and performs table mapping One DataAdapter can manage one database relation The result collection can have any level of complexity The DataAdapter is also responsible for issuing requests for new connections and terminating connections after it obtains the data

DataReader Provides a live connection to the database However, it only provides a means

of reading the database In addition, the DataReader cursor works only in the forward direction This is the object to use to perform a fast retrieval of a local table when there is no need to update the database The DataReader blocks subsequent DataAdapters and associated Connection objects, so it’s important to close the DataReader immediately after using it

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