Hướng dẫn học Microsoft SQL Server 2008 part 21 pdf

Nielsen p02.tex V4 - 07/21/2009 12:38pm Page 165Manipulating Data with Select IN THIS PART Chapter 8 Introducing Basic Query Flow Chapter 9 Data Types, Expressions, and Scalar Functions

Trang 1

Nielsen c07.tex V4 - 07/23/2009 9:03pm Page 162

Part I Laying the Foundation

The script first loads all of the DLLs needed for SMO and the snap-ins (code and commands that

extend PowerShell’s capabilities), and then it loads the snap-ins Once this is done, all of the SQL Server

provider functionality is available, and this script can be run against PowerShell 1.0 or PowerShell 2.0

when it is available

The SQL PSDrive – SQLSERVER:

Native PowerShell provides the ability to navigate not only the disk file system, but also the system

reg-istry as though it were a file system (This is expected behavior for a shell environment, as Unix shell

systems treat most everything as a file system as well.) The SQL Server provider adds a new PowerShell

drive, also referred to as a PSDrive, called SQLSERVER: TheSet-Locationcmdlet (usually aliased

ascd) is used to change to the SQLSERVER: drive and then SQL Server can be navigated like the file

system

There are four main directories under SQLSERVER: — SQL, SQLPolicy, SQLRegistration, and

DataCol-lection:

■ The SQL folder provides access to the database engine, SQL Server Agent, Service Broker, and

Database Mail, all using the various SMO DLLs

■ The SQLPolicy folder provides access to policy-based management using the DMF and Facets

DLLs

■ The SQLRegistration folder enables access to the Registered Servers (and the new Central

Management Server feature of SQL Server 2008)

■ The DataCollection folder enables access to the Data Collector objects provided with the

Management Data Warehouse feature of SQL Server 2008

You can browse the SQLSERVER file system just like a disk file system Issuing the commandcd SQL

(orSet-Location SQL) and running theGet-ChildItemcmdlet returns the local server and any

other servers that may have been recently accessed from the PowerShell session Changing to the local

server and runningGet-ChildItemreturns the names of the SQL Server instances installed on that

server Changing to one of the instances and runningGet-ChildItemreturns the collections of objects

available to that server, such as BackupDevices, Databases, Logins, and so on Changing to the Databases

collection and runningGet-ChildItemreturns the list of user databases, along with some of the

database properties The results will look something like Figure 7-6

SQL cmdlets

The SQL Server PowerShell snap-in also provides new cmdlets specific for use with SQL Server

The majority of administrative functions are managed using SMO, and data access is managed using

ADO.NET, as mentioned before, so no cmdlets were needed for these functions Some functions are just

easier using cmdlets, so they were provided They include the following:

■ Invoke-Sqlcmd

■ Invoke-PolicyEvaluation

■ Encode-SqlName

■ Decode-SqlName

■ Convert-UrnToPath

Trang 2

Scripting with PowerShell 7

FIGURE 7-6

Navigating the SQL Server ‘‘filesystem’’

The first,Invoke-Sqlcmd, takes query text and sends it to SQL Server for processing Rather than set

up the structures in ADO.NET to execute queries, theInvoke-Sqlcmdcmdlet returns results from a

query passed in as a parameter or from a text file, which provides a very easy way to get data out of

SQL Server It can perform either a standard Transact-SQL query or an XQuery statement, which

pro-vides additional flexibility

TheInvoke-PolicyEvaluationcmdlet uses the Policy-based Management feature of SQL Server

2008 It evaluates a set of objects against a policy defined for one or more servers to determine whether

or not the objects comply with the conditions defined in the policy It can also be used to reset

object settings to comply with the policy, if that is needed Lara Rubbelke has a set of blog posts on

using this cmdlet athttp://sqlblog.com/blogs/lara rubbelke/archive/2008/06/19/

evaluating-policies-on-demand-through-powershell.aspx

The character set used by SQL Server has a number of conflicts with the character set allowed by

PowerShell For example, a standard SQL Server instance name is SQLTBWS\INST01 The backslash

embedded in the name can cause PowerShell to infer a file system directory and subdirectory, because it

uses that character to separate the elements of the file system TheEncode-SqlNamecmdlet converts

strings acceptable to SQL Server into strings acceptable by PowerShell For example, the instance name

SQLTBWS\INST01 would be converted by this cmdlet into SQLTBWS%5CINST01

163

www.getcoolebook.com

Trang 3

Part I Laying the Foundation

TheDecode-SqlNamecmdlet does the exact opposite ofEncode-SqlName: It converts the

PowerShell-acceptable string of SQLTBWS%5CINST01 back to SQLTBWS\INST01

Because SMO uses Uniform Resource Names (URN) for its objects, a cmdlet is provided to convert

those URN values to path names, which can be used in aSet-Locationcmdlet — for example, to

navigate through the SQL Server objects The URN for theHumanResources.Employeetable in

AdventureWorks2008on SQLTBWS\INST01 is as follows:

Server[@Name=’SQLTBWS\INST01’]\Database[@Name=’AdventureWorks2008’]\

Table[@Name=’Employee’ and @Schema=’HumanResources’]

Converting that to a path usingConvert-UrnToPathwould yield the following:

SQLSERVER:\SQL\SQLTBWS\INST01\Databases\AdventureWorks2008\

Tables\HumanResources.Employee

Summary

After looking at the basics of PowerShell and exploring a few ways to get some interesting information

about servers, this chapter reviewed a script to provide information about each server you manage Then

it examined some of the structures in SQL Server Management Objects (SMO) and some scripts to

per-form basic administrative tasks This chapter also looked at a couple of scripts to extract data from SQL

Server, because that’s a common request from businesspeople Finally, this chapter took a quick look at

the features in SQL Server 2008 to make PowerShell an integral part of the SQL Server toolset

Much more can be explored with PowerShell, but this will provide a starting point Automation enables

administrators to do more in less time and provide more value to the companies that employ them

PowerShell is a powerful way to automate most everything an administrator needs to do with SQL

Server

Trang 4

Nielsen p02.tex V4 - 07/21/2009 12:38pm Page 165

Manipulating Data

with Select

IN THIS PART

Chapter 8

Introducing Basic Query Flow

Chapter 9

Data Types, Expressions, and Scalar Functions

Chapter 10

Merging Data with Joins and Unions

Chapter 11

Including Data with Subqueries and CTEs

Chapter 12

Aggregating Data

Chapter 13

Windowing and Ranking

Chapter 14

Projecting Data Through Views

Chapter 15

Modifying Data

Chapter 16

Modification Obstacles

SQL is like algebra in action

The etymology of the word ‘‘algebra’’ goes back to the Arabic word ‘‘al-jabr,’’

meaning ‘‘the reunion of broken parts,’’ or literally, ‘‘to set a broken bone.’’

Both algebra and SQL piece together fragments to solve a problem

I believe select is the most powerful word in all of computer science.

Because select is so common, it’s easy to take it for granted, but no keyword

in any programming language I can think of is as powerful and flexible

Select can retrieve, twist, shape, join, and group data in nearly any way

imaginable, and it’s easily extended with the insert, update, delete (and now

merge!) commands to modify data

Part II begins by exploring the basic logical query flow and quickly digs

deeper into topics such as aggregate queries, relational division, correlated

subqueries, and set-difference queries I’ve devoted 15 chapters to the select

command and its variations because understanding the multiple options and

creative techniques available with queries is critical to becoming a successful

SQL Server developer, DBA, or architect

Please don’t assume that Part II is only for beginners These 15 chapters

present the core power of SQL Part IX explores optimization strategies, and

it may be tempting to go straight there for optimization ideas, but the

second strategy of Smart Database Design is using good set-based code

Here are nine chapters describing how to optimize your database by writing

better queries

If SQL Server is the box, Part II is about being one with the box

Trang 5

Nielsen p02.tex V4 - 07/21/2009 12:38pm Page 166

Trang 6

Introducing Basic

Query Flow

IN THIS CHAPTER

Logical flow of the query Restricting the result set Projecting data

Specifying the sort order

SQL is the romance language of data, but wooing the single correct answer

from gigabytes of relational data can seem overwhelming until the logical

flow of the query is mastered

One of the first points to understand is that SQL is a declarative language This

means that the SQL query logically describes the question to the SQL Query

Optimizer, which then determines the best method to physically execute the

query As you’ll see in the next eight chapters, there are often many ways of

stating the query, but each method could be optimized to the same query

execution plan This means you are free to express the SQL query in the way

that makes the most sense and will be the easiest to maintain In some cases, one

method is considered cleaner or faster than another: I’ll point those instances

out as well

SQL queries aren’t limited toSELECT The four Data Manipulation Language

(DML) commands,SELECT,INSERT,UPDATE, andDELETE, are sometimes

taught as four separate and distinct commands However, I see queries as a single

structural method of manipulating data; in other words, it’s better to think of

the four commands as four verbs that may each be used with the full power and

flexibility of the SQL

Neither are SQL queries limited to graphical interfaces Many SQL developers

who came up through the ranks from Access and who have built queries using

only the Access query interface are amazed when they understand the enormous

power of the full SQL query

This chapter builds a basic single table query and establishes the logical query

execution order critical for developing basic or advanced queries With this

foun-dation in place, the rest of Part II develops the basicSELECTinto what I believe

is the most elegant, flexible, and powerful command in all of computing

167

Trang 7

Part II Manipulating Data with Select

Understanding Query Flow

One can think about query flow in four different ways Personally, when I develop SQL code, I imagine

the query using the logical flow method Some developers think through a query visually using the

lay-out of SQL Server Management Studio’s Query Designer The syntax of the query is in a specific fixed

order:SELECT –FROM –WHERE – GROUP BY– HAVING –ORDER BY To illustrate the declarative

nature of SQL, the fourth way of thinking about the query flow — the actual physical execution of the

query — is optimized to execute in the most efficient order depending on the data mix and the available

indexes

Syntactical flow of the query statement

In its basic form, theSELECTstatement tells SQL Server what data to retrieve, including which

columns, rows, and tables to pull from, and how to sort the data

Here’s an abbreviated syntax for theSELECTcommand:

SELECT [DISTINCT][TOP (n)] *, columns, or expressions

[FROM data source(s)]

[JOIN data source

ON condition](may include multiple joins) [WHERE conditions]

[GROUP BY columns]

[HAVING conditions]

[ORDER BY Columns];

TheSELECTstatement begins with a list of columns or expressions At least one expression is

required — everything else is optional The simplest possible validSELECTstatement is as follows:

SELECT 1;

TheFROMportion of theSELECTstatement assembles all the data sources into a result set, which is

then acted upon by the rest of theSELECTstatement Within theFROMclause, multiple tables may be

referenced by using one of several types of joins

When noFROMclause is supplied, SQL Server returns a single row with values (Oracle requires aFROM

DUALto accomplish the same thing.)

TheWHEREclause acts upon the record set assembled by theFROMclause to filter certain rows based

upon conditions

Aggregate functions perform summation-type operations across the data set TheGROUP BYclause can

group the larger data set into smaller data sets based on the columns specified in theGROUP BYclause

The aggregate functions are then performed on the new smaller groups of data The results of the

aggre-gation can be restricted using theHAVINGclause

Finally, theORDER BYclause determines the sort order of the result set

Trang 8

Introducing Basic Query Flow 8

A graphical view of the query statement

SQL Server Management Studio includes two basic methods for constructing and submitting queries:

Query Designer and Query Editor Query Designer offers a graphical method of building a query,

whereas Query Editor is an excellent tool for writing SQL code or ad hoc data retrieval because there are

no graphics to get in the way and the developer can work as close to the SQL code as possible

From SQL Server’s point of view, it doesn’t matter where the query originates; each statement is

evalu-ated and processed as a SQL statement

When selecting data using Query Designer, the SQL statements can be entered as raw code in the third

pane, as shown in Figure 8-1 The bottom pane displays the results in Grid mode or Text mode and

displays any messages The Object Browser presents a tree of all the objects in SQL Server, as well as

templates for creating new objects with code

FIGURE 8-1

The Query Designer can be used to graphically create queries

If text is selected in the Query Editor, then only the highlighted text is submitted to SQL

Server when the Execute command button or the F5 key is pressed This is an excellent

way to test single SQL statements or portions of SQL code.

169

Trang 9

Part II Manipulating Data with Select

Though it may vary depending on the user account settings, the default database is probably the master database Be sure to change to the appropriate user database using the database selector combo box in the toolbar, or the USE database command.

The best solution is to change the user’s default database to a user database and avoid master

altogether.

Logical flow of the query statement

The best way to think through a SQL DML statement is to walk through the query’s logical flow (see

Figure 8-2 Because SQL is a declarative language, the logical flow may or may not be the actual

phys-ical flow that SQL Server’s query processor uses to execute the query Nor is the logphys-ical flow the same as

the query syntax Regardless, I recommend thinking through a query in the following order

FIGURE 8-2

A simplified view of the logical flow of the query showing how data moves through the major clauses

of the SQLselectcommand

Data Source(s)

From

Expr(s)

Order

Here’s a more detailed explanation of the logical flow of the query Note that every step except step 4 is

optional:

1 [From]: The query begins by assembling the initial set of data, as specified in theFROM portion of theSELECTstatement (Chapter 10, ‘‘Merging Data with Joins and Unions,’’ and Chapter 11, ‘‘Including Data with Subqueries and CTEs,’’ discuss how to build even the most complexFROMclauses.)

2 [Where]: The filter process is actually theWHEREclause selecting only those rows that meet the criteria

3 [Aggregations]: SQL can optionally perform aggregations on the data set, such as finding the

average, grouping the data by values in a column, and filtering the groups (see Chapter 12,

‘‘Aggregating Data’’)

4 Column Expressions: TheSELECTlist is processed, and any expressions are calculated (covered in Chapter 9, ‘‘Data Types, Expressions, and Scalar Functions,’’ and Chapter 11,

‘‘Including Data with Subqueries and CTEs’’)

5 [Order By]: The resulting rows are sorted according to theORDER BYclause

6 [Over]: Windowing and ranking functions can provide a separately ordered view of the results

with additional aggregate functions

7 [Distinct]: Any duplicate rows are eliminated from the result set.

Trang 10

Introducing Basic Query Flow 8

8 [Top]: After the rows are selected, the calculations are performed, and the data is sorted into

the desired order, SQL can restrict the output to the top few rows

9 [Insert, Update, Delete]: The final logical step of the query is to apply the data modification

action to the results of the query These three verbs are explained in Chapter 15, ‘‘Modifying

Data.’’

10 [Output]: The inserted and deleted virtual tables (normally only used with a trigger) can be

selected and returned to the client, inserted into a table, or serve as a data source to an outer

query

11 [Union]: The results of multiple queries can be stacked using a union command (see

Chapter 10, ‘‘Merging Data with Joins and Unions’’)

As more complexity has been added to the SQL SELECTcommand over the years, how to think

through the logical flow has also become more complex In various sources, you’ll find minor differences

in how SQL MVPs view the logical flow That’s OK — it’s just a way to think through a query, and this

is the way I think through writing a query

As you begin to think in terms of the SQLSELECTstatement, rather than in terms of the graphical user

interface, understanding the flow of SELECTand how to read the query execution plan will help you

think through and develop difficult queries

Physical flow of the query statement

SQL Server will take theSELECTstatement and develop an optimized query execution plan, which may

not be in the execution order you would guess (see Figure 8-3) The indexes available to the SQL Server

Query Optimizer also affect the query execution plan, as explained in Chapter 64, ‘‘Indexing Strategies.’’

The rest of this chapter walks through the logical order of the basic query

From Clause Data Sources

The first logical component of a typical SQLSELECTstatement is theFROMclause In a simple SQL

SELECTstatement, theFROMclause contains a single table However, theFROMclause can also combine

data from multiple sources and multiple types of data sources The maximum number of tables that may

be accessed within a single SQLSELECTstatement is 256

TheFROMclause is the foundation of the rest of the SQL statement In order for a table column to be in

the output, or accessed in theWHEREconditions, or in theORDER BY, it must be in theFROMclause

Possible data sources

SQL is extremely flexible and can accept data from seven distinctly different types of data sources within

theFROMclause:

■ Local SQL Server tables

■ Subqueries serving as derived tables, also called subselects or in-line views, are explained in

Chapter 11, ‘‘Including Data with Subqueries and CTEs.’’ Common table expressions (CTEs)

are functionally similar to subqueries but may be referenced multiple times within the query

171

Định dạng
Số trang	10
Dung lượng	1,44 MB