Hướng dẫn học Microsoft SQL Server 2008 part 82 potx

LINQ is a standard, unified query experience across different data sources, built directly into your .NET-supported programming language.. Whether a sequence of values is returned or a s

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Nielsen c33.tex V4 - 07/21/2009 2:06pm Page 772

Part V Data Connectivity

When tracking incremental changes, Sync Services uses net change tracking for peer-to-peer

synchroniza-tion This means that the latest version is synchronized and all changes to a given row are applied in

order at each peer

Peer-to-peer synchronization can be compared in many ways to a combination of offline data-set

technology and replication Sync Services provides a programming model like that of offline datasets

but with a feature set like that of replication As stated in the previous section, replication is a database

administrator function, but via Sync Services, developers can have the same synchronization support

through the many available synchronization classes, enabling developers to synchronize with other data

store types

Like the offline scenario, the choice between using peer-to-peer transaction replication versus using Sync

Services for peer-to-peer synchronization, it comes down to need Peer-to-peer transactional replication

is more of an administrator responsibility, but it does provide a highly available and scalable system and

requires almost no coding

Sync Services, however, is for developers It allows for synchronization of different transports or services,

and provides synchronization capabilities with data sources other than a database Sync Services is the

mechanism for moving beyond just replicating schema and data from one database to another

From an architecture standpoint, the peer-to-peer scenario has each peer containing its own application

code and Sync Services code to initiate and facilitate synchronization This functionality is provided via

the Sync Services peer-to-peer API During a typical synchronization, a pair of peers is synchronized

For example, given a scenario in which three peers need to be synchronized, synchronization could take

place like this: Peer1 and peer2 are synchronized, then peer1 and peer3, and then finally peer2 and

peer3 In each synchronization, one peer acts as the local peer, and one peer acts as the remote peer

However, this scenario might seem a bit redundant, as it is not necessary for each and every peer to be

synchronized In the earlier example, peer1 and peer2 could be synchronized, then peer2 and peer3

Peer1 and peer3 would be synchronized via peer2

As for the components used in a peer-to-peer synchronization, Figures 33-12 and 33-13 show which

components are used in a two-tier and n-tier architecture, respectively.

FIGURE 33-12

A two-tier architecture in peer-to-peer synchronization

DbSyncAdapter DbSyncAdapter

Computer

SyncOrchestrator

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Sync Framework 33

FIGURE 33-13

Ann-tier architecture in peer-to-peer synchronization with a proxy

DbSyncAdapter DbSyncAdapter

Local Computer Remote Computer

SyncOrchestrator Proxy Service

In a two-tier architecture, all the services exist on the local peer, as shown in Figure 33-12

In an n-tier architecture, a proxy and service are also introduced, and some of the services reside on the

remote computer, as shown in Figure 33-13

The n-tier architecture requires a service and a transport system which provides communication between

a proxy on the local computer and a service on the remote computer The difference between

peer-to-peer and client-server synchronization is that the proxy is not part of the Sync Framework API This

proxy must be written by the developer and must be derived from theKnowledgeSyncProvider

class

TheSyncOrchestratorin a peer-to-peer synchronization is responsible for the following:

■ Storing table information on the peer

■ Applying incremental changes to the peer database

■ Detecting conflict changes

■ Assisting applications in obtaining changes that occurred on the peer since the last

synchro-nization

By now it should be fairly clear that using Sync Services for ADO.NET 2.0 provides a very powerful yet

flexible synchronization mechanism There are many options to consider What makes Sync Services

great is simply how comprehensive it is for building offline and collaboration applications

Summary

This chapter provided a look at the Microsoft Sync Framework and Sync Services for ADO.NET 2.0

The chapter began by providing a brief and simple example demonstrating how easy it is to use this

great technology It then jumped feet first into first an overview, and then the architecture, of the Sync

Framework

That discussion was followed by an overview and architectural discussion about one of the Sync

Frame-work components, Sync Services, and why it should be considered when architecting offline solutions

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LINQ

IN THIS CHAPTER

Overview and history of LINQ Understanding LINQ

LINQ standard query operators Syntax options

LINQ to SQL LINQ to XML LINQ to Dataset LINQ to Entities

Let’s get right to the point I often see code like this:

string ConnectionString = @"Data Source=(local);

Initial Catalog = Adventureworks;UID=username;PWD=password";

using (SqlConnection conn = new

SqlConnection(ConnectionString))

{

conn.Open();

SqlCommand cmd = conn.CreateCommand();

cmd.CommandType = CommandType.Text;

cmd.CommandText = "SELECT LastName, FirstName,

MidleName FROM Person.Contact";

using (SqlDataReader rdr = cmd.ExecuteReader())

{

// do something

}

Maybe you have also seen code like this or have even done it yourself, but at

some point in your career one of the two have happened Given the preceding

code, ask yourself two questions First, will it compile when run? Second, if so,

will it run successfully? Set aside for a minute the fact that there is noWHERE

clause and ignore the lack of encapsulation That is not the intent of this example

What is important are the two questions

First, yes, it will compile Second, when it runs it will fail It will fail because the

column ‘‘MidleName’’ should actually be ‘‘MiddleName.’’ The problem is that you

need to run the program in order to catch this Even worse, if your program does

error, where do you look? (That is, the developer was unaware that the column

name was misspelled.) Debugging an error such as this is time consuming and

unproductive Equally, the example is ‘‘unfriendly’’ on several levels First,

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two languages are being combined, in this case, C# and T-SQL The problem is that the T-SQL language

is not understood in the context of NET

Another nuisance is the fact that there is a different set of technologies for the different data sources

(SQL databases, XML, collections, in-memory objects, and so on) that developers work with on a daily

basis For example, you typically use ADO.NET to work with databases, but you might use XQuery,

XPath, or some of the XML classes built into the NET Framework to work with XML

Those are the two issues: Because queries against data are often written as strings embedded in code, the

result is no compile-time checking or IntelliSense support, as well as difficulty debugging; and multiple

technologies (different query languages) for different data sources (SQL, XML, etc.)

Wouldn’t it be nice if there were a way to solve both of these issues? Even better, wouldn’t it be nice if

there were a way to solve both of these issues with a single technology?

Luckily, there is an answer: Microsoft LINQ This chapter provides a brief history and overview of

LINQ, and then looks at the different LINQ providers, including examples that demonstrate how to

use them

LINQ Overview

Before looking at LINQ’s current incarnation, it would be wise to first look at its history Microsoft

LINQ has been in the works for well over five years LINQ (and other LINQ providers such as LINQ

to SQL) has roots that extend to previous projects, which have been growing and evolving for quite

some time

LINQ goes all the way back to a previous Microsoft project named C-Omega (or C) C-Omega, in its

fullness, really contained more than what shows up in LINQ, as it contained functionality that truly

experimented with using integrated queries (a mix of C# and other query languages such as SQL and

XQuery) C was initially released to the public as a preview in 2004, and it quickly caught the attention

of other Microsoft people (such as Anders Hejlsberg) who then took that same concept to C# and other

languages

Yet, as good as C-Omega was, LINQ now contains a lot of what C contained, plus many additional

fea-tures and functionality It could be said that LINQ ‘‘leapfrogged’’ over C Those familiar with Microsoft’s

ORM history know that LINQ to SQL is not Microsoft’s first endeavor at object-relational mapping The

roots of LINQ to SQL can actually be traced to something called ObjectSpaces dating back to 2001 In

simple terms, ObjectSpaces is a set of data access APIs that enable data to be treated as objects,

regard-less of the source of the data

This story has both a happy ending and a sad ending The sad ending is that ObjectSpaces was never

officially released, simply because ObjectSpaces depended on WinFS and when Microsoft made the

announcement that WinFS would not make it into the first release of Windows Vista, that was the end

of ObjectSpaces Happily, however, Microsoft learned a ton about ORM from ObjectSpaces and applied

that knowledge to LINQ to SQL, the result being a well thought-out and solid ORM product Is it the

ORM product to end all ORM products? No, but even for a first release, LINQ to SQL is surely nothing

to sneeze at

In the same category is a product called Nordic, developed by Paul Nielsen, which is a T-SQL-based

O/R DBMS fac¸ade It is a nice utility that uses code-gen to emulate an object-database It includes

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polymorphism, inheritance, inheritable workflow states, and inheritable complex expressions Moreover,

according to Paul, it’s fast This nice tool can be found on CodePlex atwww.codeplex.com/Nordic

What Is LINQ?

LINQ stands for language-integrated query In essence, this means that developers have at their disposal

a query language integrated directly into their NET programming of choice (such as C# and Visual

Basic), giving them general-purpose query capabilities

LINQ is a standard, unified query experience across different data sources, built directly into your

.NET-supported programming language LINQ brings powerful query facilities to the NET Framework

lan-guages, such as C# and VB.NET Microsoft accomplished this by turning query set operations,

trans-forms, and constructs into high-level concepts (on the same level of classes and objects) within the NET

Framework LINQ makes a query a first-class construct within C# and Visual Basic

Moreover, LINQ addresses the very large mismatch between programming languages and data sources

such as databases, XML, and in-memory objects The entire goal of LINQ is to make the interaction

between objects and data sources easier and simpler, enabling developers to interact with the different

data sources (such as SQL and XML), because there is a disconnect between the data sources and the

programming languages that communicate with (that is, work with) each of them

As stated earlier, LINQ is a ‘‘unified’’ query experience This means that it unifies data access regardless

of the source of data Simply stated, LINQ bridges the gap between the world of data and the world of

programming

Prior to LINQ, developers had to learn different technologies to query different data sources No more!

LINQ gives developers a new way of querying different types of data using strongly typed queries

com-mon across many different data sources The result is improved developer productivity along with

LINQ-specific IntelliSense in the Visual Studio IDE and compile-time error checking!

That’s right Because developers are now working with data in an object-oriented fashion, they are also

working in a strongly typed environment The benefits of this include compile-time error checking, not

runtime error checking In addition, developers can now write queries using Visual Studio’s IntelliSense,

driving the experience toward a more declarative programming model

Because LINQ makes a query a first-class construct within the programmer’s development language,

developers can now write and execute queries against strongly typed objects using keywords, or

operators These keywords, called standard query operators, are discussed in the next section.

Who should use LINQ? The real question is who shouldn’t use LINQ? LINQ is targeted toward

appli-cation developers who want a new way to query data using strongly typed queries and strongly typed

results It is for developers seeking commonality across a number of different data sources, including but

not limited to relational databases It is for developers who want improved productivity — and who like

using really cool technology

Standard Query Operators

Standard query operators are essentially methods that form the LINQ pattern They provide the query

capabilities, including filtering, projection, sorting, and more A large portion of these methods that

make up the standard query operators operate on sequences, objects that have implemented the

IEnumerable<T>orIQueryable<T>interface These two interfaces are what provide the querying

and iteration for queries over a non-generic collection

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There are two sets of standard query operators that implement the two interfaces mentioned above

The key to remember when working with the standard query operators is that regardless of the

set, each method of the standard query operators is a static member of theEnumerableand

Queryableclasses, meaning they can be called using either static method syntax or instance method

syntax

The standard query operators also vary in their execution timing This timing depends on the results

of the query Whether a sequence of values is returned or a singleton value is returned determines

when the query is executed Methods are executed immediately if they return a singleton value;

otherwise, the query execution is deferred and an enumerable object is returned if the method returns a

sequence

Table 34-1 lists the 53 standard query operators, grouped according to their functionality

Query expression syntax

To understand the flow of LINQ query syntax, compare it to standard T-SQL syntax If you have written

any T-SQL, you know the basic T-SQL query syntax and how it is written For instance, a simple query

looks like the following:

SELECT FirstName, LastName FROM Person.Contact

TABLE 34-1

Standard Query Operators Functionality Standard Query Operator Description

Sorting OrderBy Sorts values in ascending order

OrderByDescending Sorts values in descending order ThenBy Applies a secondary sort in ascending order ThenByDescending Applies a secondary sort in descending order Reverse Sorts the elements of a collection in reverse order Set Distinct Removes duplicate values from a collection

Except Returns the differences between two sets (elements in

one collection that do not exist in a second collection) Intersect Returns the matches between two sets (elements that

appear in two separate collections) Union Returns the union of two sets (unique elements that

appear in either of two collections)

continued

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TABLE 34-1 (continued )

Functionality Standard Query Operator Description

Filtering OfType Selects values from a sequence based on their ability to

be cast to a specified type Where Selects values from a sequence based on a predicate

function Quantifier All Determines whether all the elements in a sequence meet

a condition Any Determines whether any of the elements in a sequence

meet a condition Contains Determines whether a sequence contains a specified

element Projection Select Projects values based on a transform function

SelectMany Projects sequences of values based on a transform

function and then combines them into a single sequence Partitioning Skip Skips elements up to a specified position in a sequence

SkipWhile Skips elements based on a predicate function until an

element does not satisfy the condition Take Takes elements up to a specified position in a sequence

TakeWhile Takes elements based on a predicate function until an

element does not satisfy the condition Join Join Joins two sequences based on key selector functions

GroupJoin Joins two sequences based on key selector, grouping the

matches for each element Grouping GroupBy Groups elements that share a common attribute

ToLookup Inserts elements into a Lookup(TKey, TElement)

Generation DefaultIfEmpty Replaces an empty collection with a default singleton

collection Empty Returns an empty collection

Range Generates a collection that contains a sequence of

numbers Repeat Generates a collection that contains one repeated value

Equality SequenceEqual Determines whether two sequences are equal by

comparing elements in a pair-wise manner

continued

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Element ElementAt Returns the element at a specified index in a collection

ElementAtOrDefault Returns the element at the specified index in a

collection, or the first element that satisfies a condition First Returns the first element of a collection, or the first

element that satisfies a condition FirstOrDefault Returns the first element of a collection, or the first

element that satisfies a condition A default value is returned if the specified element does not exist

Last Returns the last element of a collection or the last

element that satisfies a condition LastOrDefault Returns the last element of a collection or the last

element that satisfies a condition A default value is returned if the specified element does not exist in the collection

Single Returns the only element of a collection or the only

element that satisfies a condition SingleOrDefault Returns the only element of a collection or the only

element that satisfies a condition A default value is returned if the specified element does not exist in the collection

Conversion AsEnumerable Returns the input types as IEnumerable(T)

AsQueryable Converts a generic IEnumerable to a generic

IQueryable Cast Casts the elements of a collection to a specified type OfType Filters values based on their ability to be cast to a

specified type ToArray Converts a collection to an array Forces query

execution

ToDictionary Puts elements into a Dictionary(TKey, TValue)

Forces query execution

ToList Converts a collection to a List(T) Forces query

execution

ToLookup Puts elements into a Lookup(TKey, TValue)as a

one-to-many dictionary Forces query execution

continued

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Concatenation Concat Concatenates two sequences into a single sequence

Aggregation Aggregate Performs a custom aggregation operation on the values of

a collection Average Calculates the average value of a collection of values Count Counts the elements in a collection; optionally, only

those elements that satisfy a predicate expression LongCount Counts the elements in a large collection; optionally,

only those elements that satisfy a predicate expression Max Determines the maximum value in a collection Min Determines the minimum value in a collection Sum Calculates the sum of the values in a collection

This example queries thePerson.Contacttable in the AdventureWorks database and returns the

FirstNameandLastNamecolumns for each row in the table Because that’s so simple, the following

example adds a secondary table, and then applies a filter and a sort:

SELECT E.EmployeeID,C.FirstName, C.LastName

FROM Person.Contact AS C

INNER JOIN HoumanResources.Employee AS E ON C.ContactID = E.ContactID

WHERE E.EmployeeID < 100

ORDER BY C.LastName

This is the syntax with which all T-SQL developers are familiar At the very minimum, the query begins

with aSELECTclause, which specifies the columns you want to be returned by the query, followed

by aFROMclause, which lists the tables and/or views containing the columns identified in theSELECT

clause

The query could include one or more joins such as anINNER JOINorOUTER JOIN, followed by some

filtering using theWHEREclause and possibly aGROUP BYorHAVINGclause, and quite possibly some

ordering using theORDER BYclause

How many developers have really stopped to think about how SQL Server processes these queries? Does

it execute the query from top to bottom, starting with the SELECTclause and working its way down?

One might assume that, but that is not how a query is processed in SQL Server at all SQL Server

logi-cally processes a query in the order indicated here by the number in parentheses:

(8) SELECT

(9) TOP

(1) FROM

(3) JOIN

(2) ON

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