Hướng dẫn học Microsoft SQL Server 2008 part 25 pot

Because null values are unknown, the result of any expression that includes null will also have a value that is unknown.. Consider this simple test which proves that null does not equal

Case expressions

SQL Server’sCASEexpression is a flexible and excellent means of building dynamic expressions

If you’re a programmer, no doubt you use the case command in other languages The SQLCASE

expression, however, is different It’s not used for programmatic flow of control, but rather to logically

determine the value of an expression based on a condition

Best Practice

When programmers write procedural code, it’s often because part of the formula changes depending on

the data To a procedural mind-set, the best way to handle this is to loop through the rows and use

multiple IF statements to branch to the correct formula However, using a CASE expression to handle the

various calculations and executing the entire operation in a single query enables SQL Server to optimize

the process and make itdramatically faster

Because thecaseexpression returns an expression, it may be used anywhere in the SQL DML

statement (SELECT,INSERT,UPDATE,DELETE) where an expression may be used, including column

expressions, join conditions, where conditions, having conditions, in theORDER BY, or even embedded

in a longer expression A case expression can even be used mid-expression to create a dynamic formula

– very powerful

TheCASEstatement has two forms, simple and searched, described in the following sections

Simple case

With the simpleCASE, the variable is presented first and then each test condition is listed However,

this version ofCASEis limited in that it can perform only equal comparisons TheCASEexpression

sequentially checks theWHENconditions and returns theTHENvalue of the first trueWHENcondition

In the following example, based on the OBXKites database, oneCustomerTypeis the default for new

customers and is set totruein theIsDefaultcolumn TheCASEexpression compares the value in

the default column with each possible bit setting and returns the character string‘default type’or

‘possible’based on the bit setting:

USE OBXKites;

SELECT CustomerTypeName, CASE IsDefault WHEN 1 THEN ‘default type’

WHEN 0 THEN ‘possible’

ELSE ‘-’

END AS AssignStatus FROM CustomerType;

CustomerTypeName AssignStatus

-

TheCASEexpression concludes with an end and an alias In this example, theCASEexpression

evalu-ates theIsDefaultcolumn, but produces the AssignStatus column in the SQLSELECTresult set

Be careful if you useNULLin a simpleCASE This translates literally to ‘‘=NULL’’ and not to ‘‘IS NULL’’

You can get unintended results if you are not careful

Boolean case

The Boolean form of case (called the searched case in BOL) is more flexible than the simple form in

that each individual case has its own Boolean expression Therefore, not only can eachWHENcondition

include comparisons other than=, but the comparison may also reference different columns:

SELECT

CASE

WHEN 1<0 THEN ‘Reality is gone.’

WHEN CURRENT_TIMESTAMP = ‘20051130’

THEN ‘David gets his driver’’s license.’

WHEN 1>0 THEN ‘Life is normal.’

END AS RealityCheck;

Following is the result of the query when executed on David’s sixteenth birthday:

RealityCheck

-David gets his driver’s license

As with the simple case, the first trueWHENcondition halts evaluation of the case and returns theTHEN

value In this case (a pun!), if 1 is ever less than 0, then theRealityCheckcase will accurately report

‘reality is gone.’When my son turns 16, theRealityCheckwill again accurately warn us of

his legal driving status If neither of these conditions is true, and 1 is still greater than 0, then all is well

with reality and‘Life is normal.’

The point of the preceding code is that the searchedCASEexpression offers more flexibility than the

simpleCASE This example mixed various conditional checks (<,=,>), and differing data was checked

by theWHENclause

The BooleanCASEexpression can handle complex conditions, including BooleanANDandOR

opera-tors The following code sample uses a batch to set up theCASEexpression (including T-SQL variables,

which are explained in Chapter 21, ‘‘Programming with T-SQL’’), and theCASEincludes anANDand a

BETWEENoperator:

DECLARE @b INT, @q INT;

SET @b = 2007;

SET @q = 25;

SELECT CASE WHEN @b = 2007 AND @q BETWEEN 10 AND 30 THEN 1 ELSE NULL

END AS Test;

Result:

Test -1

Working with nulls

The relational database model represents missing data using null Technically, null means ‘‘value absent’’

and it’s commonly understood to mean ‘‘unknown.’’ In practice, null can indicate that the data has not

yet been entered into the database or that the column does not apply to the particular row

Because null values are unknown, the result of any expression that includes null will also have a value

that is unknown If the contents of a bank account are unknown, and its funds are included in a

port-folio, then the total value of the portfolio is also unknown The same concept is true in SQL, as the

fol-lowing code demonstrates Phil Senn, a database developer, puts it this way: ‘‘Nulls zap the life out of

any other value.’’

SELECT 1 + NULL;

Result:

NULL Because nulls have such a devastating effect on expressions, some developers detest the use of nulls

They develop their databases so that nulls are never permitted, and column defaults supply surrogate

nulls (blank, 0, or ‘n/a’) instead

Other database developers argue that an unknown value should be represented by a zero or a blank just

to make coding easier I fall into the latter camp Nulls are valuable in a database because they provide a

consistent method of identifying missing data And regardless of how missing data is represented in the

database, certain types of queries will often produce nulls in the results, so it’s worthwhile to write code

that checks for nulls and handles them appropriately

An advantage to using nulls is that SQL Server’s AVG() and COUNT(column) aggregate func-tions automatically exclude nulls from the calculation If you’re using a surrogate null (for example, I’ve seen IT shops use 0 or -999 to represent missing numeric data) then every aggregate query

must filter out the surrogate null or the results will be less than accurate.

Testing for null

Because null represents a missing value, there is no way to know whether a null is equal or unequal to

a given value, or even to another null Returning to the bank account example, if the balance of account

123 is missing and the balance of account 234 is missing, then it’s logically impossible to say whether

the two accounts have an equal or unequal balance

Consider this simple test which proves that null does not equal null:

IF NULL = NULL

SELECT ‘=’;

ELSE

SELECT ‘<> ’;

Result:

<>

Because the=and<>operators can’t check for nulls, SQL includes two special operators,ISand

IS NOT, to test for equivalence to special values, as follows:

WHERE Expression IS NULL

Repeating the simple test, theISsearch operator works as advertised:

IF NULL IS NULL

SELECT ‘Is’;

ELSE

SELECT ‘Is Not’;

Result:

Is

TheISsearch condition may be used in theSELECTstatement’sWHEREclause to locate rows with null

values Most of the Cape Hatteras Adventures customers do not have a nickname in the database The

following query retrieves only those customers with a null in theNicknamecolumn:

USE CHA2;

SELECT FirstName, LastName, Nickname

FROM dbo.Customer

WHERE Nickname IS NULL

ORDER BY LastName, FirstName;

Result:

FirstName LastName Nickname

- -

TheISoperator may be combined withNOTto test for the presence of a value by restricting the result

set to those rows whereNicknameis not null:

SELECT FirstName, LastName, Nickname

FROM dbo.Customer

WHERE Nickname IS NOT NULL

ORDER BY LastName, FirstName;

FirstName LastName Nickname - -

Handling nulls

When you are supplying data to reports, to end users, or to some applications, a null value will be less

than welcome Often a null must be converted to a valid value so that the data may be understood, or

so the expression won’t fail

Nulls require special handling when used within expressions, and SQL includes a few functions

designed specifically to handle nulls.ISNULL()andCOALESCE()convert nulls to usable values, and

NULLIF()creates a null if the specified condition is met

Using the COALESCE() function

COALESCE()is not used as often as it could (some would say should) be, perhaps because it’s not well

known It’s a very cool function.COALESCE()accepts a list of expressions or columns and returns the

first non-null value, as follows:

COALESCE(expression, expression, ) COALESCE()is derived from the Latin words co + alescre, which mean to unite toward a common end,

to grow together, or to bring opposing sides together for a common good The SQL keyword, however,

is derived from the alternate meaning of the term: ‘‘to arise from the combination of distinct elements.’’

In a sense, theCOALESCE()function brings together multiple, differing values of unknown usefulness,

and from them emerges a single valid value

Functionally,COALESCE()is the same as the following case expression:

CASE WHEN expression1 IS NOT NULL THEN expression1 WHEN expression2 IS NOT NULL THEN expression2 WHEN expression3 IS NOT NULL THEN expression3

ELSE NULL END

The following code sample demonstrates theCOALESCE()function returning the first non-null value

In this case, it’s 1+2:

SELECT COALESCE(NULL, 1+NULL, 1+2, ‘abc’);

3

COALESCE()is excellent for merging messy data For example, when a table has partial data in several

columns, theCOALESCE()function can help pull the data together In one project I worked on, the

client had collected names and addresses from several databases and applications into a single table

The contact name and company name made it into the proper columns, but some addresses were in

Address1, some were in Address2, and some were in Address3 Some rows had the second line of the

address in Address2 If the address columns had an address, then the SalesNote was a real note In

many cases, however, the addresses were in the SalesNote column Here’s the code to extract the address

from such a mess:

SELECT COALESCE(

Address1 + STR(13) + STR(10) + Address2,

Address1,

Address2,

Address3,

SalesNote) AS NewAddress

FROM TempSalesContacts;

For each row in theTempSalesContactstable, theCOALESCE()function will search through

the listed columns and return the first non-null value The first expression returns a value only if

there’s a value in both Address1 and Address2, because a value concatenated with a null produces a

null Therefore, if a two-line address exists, then it will be returned Otherwise, a one-line address in

Address1, Address2, or Address3 will be returned Failing those options, the SalesNote column will be

returned Of course, the result from such a messy source table still needs to be manually scanned and

verified

Using the ISNULL() function

The most common null-handling function isISNULL(), which is different from theIS NULLsearch

condition This function accepts a single expression and a substitution value If the source is not equal

to null, then the ISNULL()function passes the value on However, if the source is null, then the

sec-ond parameter is substituted for the null, as follows:

ISNULL(source_expression, replacement_value)

Functionally,ISNULL()is similar to the following case expression:

CASE

WHEN source_expression IS NULL THEN replacement_value

ELSE source_expression

END

The following code sample builds on the preceding queries by substituting the string (’NONE’) for a null

for customers without a nickname:

SELECT FirstName, LastName, ISNULL(Nickname,’none’)

FROM Customer

ORDER BY LastName, FirstName;

Result:

- -

If the row has a value in the Nickname column, then that value is passed though theISNULL()

func-tion untouched However, if the nickname is null for a row, then the null is handled by theISNULL()

function and converted to the value none

The ISNULL() function is specific to T-SQL, whereas NULLIF() is ANSI standard SQL.

Using the NULLIF() function

Sometimes a null should be created in place of surrogate null values If a database is polluted with n/a,

blank, or – values where it should contain nulls, then you can use theNULLIF()function to replace

the inconsistent values with nulls and clean the database

TheNULLIF()function accepts two parameters If they are equal, then it returns a null; otherwise, it

returns the first parameter Functionally,NULLIF()is the same as the following case expression:

CASE WHEN Expression1 = Expression2 THEN NULL ELSE Expression1

END The following code will convert any blanks in theNicknamecolumn into nulls The first statement

updates one of the rows to a blank for testing purposes:

UPDATE Customer SET Nickname = ‘’

WHERE LastName = ‘Adams’;

SELECT LastName, FirstName, CASE Nickname

WHEN ‘’ THEN ‘blank’

ELSE Nickname END AS Nickname, NULLIF(Nickname, ‘’) as NicknameNullIf FROM dbo.Customer

WHERE LastName IN (’Adams’, ‘Anderson’, ‘Andrews’) ORDER BY LastName, FirstName;

LastName FirstName Nickname NicknameNullIf

- - -

The third column uses a case expression to expose the blank value as ‘‘blank,’’ and indeed the

NULLIF()function converts the blank value to a null in the fourth column To test the other null

possibilities, Melissa’s Nickname was not affected by theNULLIF()function, and Debbie’s

null Nickname value is still in place

A common use ofNULLIF()prevents divide-by-zero errors The following expression will generate an

error if the variablebis zero:

a / b ∼∼ Error if b is 0, otherwise a normal division result

However, you can useNULLIF()such that if the value of thebvariable is 0, it will result in aNULL

instead of an error, as follows:

a / NULLIF(b,0) ∼∼NULL result if b is 0, otherwise a normal division result

Now with a 0 as the result instead of an error,COALESCE()can be used to replace it with something

more usable if needed

Scalar Functions

Scalar functions return a single value They are commonly used in expressions within theSELECT,

WHERE,ORDER BY,GROUP, andHAVINGclauses, or T-SQL code SQL Server includes dozens of

functions This section describes the functions I find most useful

Best Practice

Performance is as much a part of the data-schema design as it is a part of the query Plan to store the

data in the way that it will be searched by a WHERE condition, rather than depend on manipulating the

data with functions at query time While using a function in an expression in a result-set column may be

unavoidable, using a function in a WHERE condition forces the function to be calculated for every row In

addition, another bottleneck is created because using a function in a WHERE clause makes it impossible for

the Query Optimizer to use an index seek — it has to use a scan instead, resulting in much more I/O

With SQL Server 2008 you can develop three types of user-defined functions, as explained

in Chapter 25, ‘‘Building User-Defined Functions.’’

User information functions

In a client/server environment, it’s good to know who the client is Toward that end, the following four

functions are very useful, especially for gathering audit information:

■ USER_NAME(): Returns the name of the current user as he or she is known to the database

When a user is granted access to a database, a username that is different from the server login name may be assigned The results are affected by anEXECUTE AScommand, in which case the username shown is that of the impersonated user

■ SUSER_SNAME(): Returns the login name by which the user was authenticated to SQL Server

If the user was authenticated as a member of a Windows user group, then this function still returns the user’s Windows login name The results are affected by anEXECUTE AScommand,

in which case the username shown is that of the impersonated user

■ HOST_NAME(): Returns the name of the user’s workstation

■ APP_NAME(): Returns the name of the application (if set by the application itself) connected

to SQL Server, as follows:

SELECT USER_NAME() AS ‘User’, SUSER_SNAME() AS ‘Login’, HOST_NAME() AS ‘Workstation’, APP_NAME() AS ‘Application’;

Result:

User Login Workstation Application - - - -Dbo NOLI\Paul CHA2\NOLI Management Studio

Date and time functions

Databases must often work with date and time data, and SQL Server includes several useful functions for

that SQL Server stores both the data and the time in a single data type It also has types for date only,

time only, and zone-aware times

T-SQL includes several functions to return the current date and time:

■ GetDate(): Returns the current server date and time to the nearest 3 13 milliseconds, rounded to the nearest value

■ CURRENT_TIMESTAMP: The same asGETDATE()except ANSI standard

■ GetUTCDate(): Returns the current server date converted to Greenwich mean time (also known as UTC time) to the nearest 3 milliseconds This is extremely useful for companies that cross time boundaries

New to SQL Server 2008:

■ SysDateTime(): Returns the current server date and time to the nearest hundred nanoseconds

■ SysUTCDateTime(): Returns the current server date converted to Greenwich mean time to the nearest hundred nanoseconds

■ SYSDATETIMEOFFSET(): Returns aDateTimeOffsetvalue that contains the date and

time of the computer on which the instance of SQL Server is running The time zone offset is

included

■ ToDateTimeOffset(): Returns aDateTimeOffsettype

The following four SQL Server date-time functions handle extracting or working with a specific portion

of the date or time stored within a datetime column:

■ DATEADD(date portion,number,date): Returns a new value after adding the number

■ DATEDIFF(date portion,start date,end date): Returns the count of the date portion

boundaries

■ DateName(date portion,date): Returns the proper name for the selected portion of the

datetime value or its ordinal number if the selected portion has no name (the portions for

DateName()andDatePart()are listed in Table 9-2):

SELECT DATENAME(year, CURRENT_TIMESTAMP) AS "Year";

Result:

Year

-2009

This code gets the month and weekday name:

select DATENAME(MONTH,CURRENT_TIMESTAMP) as "Month",

DATENAME(WEEKDAY,CURRENT_TIMESTAMP) As "Day"

Result

Month Day

-

-February Tuesday

This code gets the month and weekday name and displays the results in Italian:

Set language Italian

select DATENAME(MONTH,CURRENT_TIMESTAMP) as "Month",

DATENAME(WEEKDAY,CURRENT_TIMESTAMP) As "Day"

Result

Month Day

-

-Febbraio Martedi

For more information about datetime , datetime2 , and other data types, refer to

Chapter 20, ‘‘Creating the Physical Database Schema.’’

The following code example assigns a date of birth to Mr Frank and then retrieves the proper

names of some of the portions of that date of birth using the DateName()function:

UPDATE Guide

SET DateOfBirth = ‘September 4 1958’

WHERE LastName = ‘Frank’;