Microsoft SQL Server 2008 R2 Unleashed- P173 pps

This technique for calculating a date interval between the current date and the year, ”1900-01-01”, and then adding the calculated number of intervals to ”1900-01-01”can be used to calcu

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convert the date value into a date you need You need to start thinking in terms of date

intervals—for example, how many date intervals it is from the current date to the date

you want to calculate, or how many date intervals it is from today to some other date,

such as ”2000-01-01”, and so on You use the DATEADDandDATEDIFFfunctions to calculate

your desired date by determining the appropriate date intervals from the current date and

then adding or subtracting intervals to arrive at the desired calculated date Understanding

how to use the various date intervals helps you more easily understand how to calculate

the desired dates

Calculating the First Day of Month

Let’s look at a method for determining the first day of the month for a given date To do

this, you start out with the initial date (In this example, you can use getdate()to work

with the current system date and time.) The next step is to figure out the number of

months between the given date and the date ”1/1/1900”

NOTE

The value “1/1/1900” is the default for a date if an empty string (’’) is used to

repre-sent a date

You can use the DATEDIFFfunction to determine the number of months from ”1/1/1900”:

select DATEDIFF(mm,’’,getdate())

go

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Now, using the number of months, you can add that result to ”1/1/1900”to obtain the

first day of the month for the given date:

select DATEADD(mm, DATEDIFF(mm,’’,getdate()), ‘’)

By adding the number of months between the given date and ”1/1/1900”to”1/1/1900”,

you are able to arrive at the first day of the current month In addition, the time portion

of the calculated date is set to ”00:00:00.000”

This technique for calculating a date interval between the current date and the year,

”1900-01-01”, and then adding the calculated number of intervals to ”1900-01-01”can be

used to calculate many different dates The next four examples use the same technique to

generate different dates based on the current date

Calculating the First Day of the Year

You can use the year interval (yy) to display the first day of the year:

select DATEADD(yy, DATEDIFF(yy,’’,getdate()), ‘’)

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Calculating the First Day of the Quarter

To calculate the first day of the current quarter, you use the quarter (qq) interval:

select DATEADD(qq, DATEDIFF(qq,’’,getdate()), ‘’)

Calculating Midnight for the Current Day

If you need to truncate the time portion for a datetimevalue so it reflects the current date

at midnight, you can use the date interval (dd) to get the midnight time stamp for the

desired date:

select DATEADD(dd, DATEDIFF(dd,’’,getdate()), ‘’)

Calculating Monday of the Current Week

You can use the week interval (wk) to calculate what date is Monday of the current week:

select DATEADD(wk, DATEDIFF(wk,’’,getdate()), ‘’)

Calculating Other Dates

As you have seen, by using simple DATEADDandDATEDIFFcalculations, you can come up

with many different dates that might be valuable All the examples so far have only

calcu-lated the number of date intervals between the current date and ”1/1/1900”and then

added the appropriate number of intervals to ”1900-01-01”to arrive at the calculated

date If you have to calculate other date values, you can use this calculation as the basis

and then add or subtract additional intervals to come up with other useful dates

For example, to calculate the last day of the previous month for a given date, you can use

the following calculation to determine the first day of the current month and subtract a

day from it:

select DATEADD(dd, -1, DATEADD(mm, DATEDIFF(mm,’’,getdate()), ‘’))

You can perform a similar calculation to determine the last day of the previous year, based

on the formula to calculate the first date of the current year for the given date:

select DATEADD(dd, -1, DATEADD(yy, DATEDIFF(yy,’’,getdate()), ‘’))

What if you need to determine the last day of the current month for a given date? One way

to do this is to calculate the first date of the next month and subtract one day from that To

calculate the first day of the next month, you can use the formula to calculate the first day

of the current month and add one to the number of intervals returned byDATEDIFFwhen

comparing the given date to”1/1/1900”to get the first day of the next month:

select DATEADD(mm, DATEDIFF(mm,’’,getdate()) + 1, ‘’)

Now that you have the first date of the next month, you simply subtract one day from it

to get the last day of the current month:

select DATEADD(dd, -1, DATEADD(mm, DATEDIFF(mm,’’,getdate()) + 1, ‘’))

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Similarly, you can modify the formula to calculate the first day of the year to return the

last day of the previous year:

select DATEADD(dd, -1, DATEADD(yy, DATEDIFF(yy,’’,getdate()) + 1, ‘’))

Now, let’s try a little more advanced calculation: the first Monday of the current month

To find this, you start with the calculation for the Monday of the current week and

modify it slightly Rather than use getdate()as the date value, you use the calculation to

get the first day of the month and add five days to it Adding five days to the first day of

the month ensures that you are in the first full week of the month (SQL Server treats

Sunday as the first day of the week, so if the first day of the month was on a Monday,

adding 5 days keeps you in the same week If the first day is Tuesday or later, adding 5

days puts you into the next week) You can use the following calculation to get the first

day of the month and add five days to it:

select DATEADD(dd, 5, DATEADD(mm, DATEDIFF(mm,’’,getdate()), ‘’))

Now, you use this expression in place of thegetdate()function in the calculation to get

the date for Monday of the current week:

select DATEADD(wk, DATEDIFF(wk,’’,

DATEADD(dd, 5, DATEADD(mm, DATEDIFF(mm,’’,getdate()), ‘’))), ‘’)

The examples presented in this chapter should give you some insight into using the

DATEADDandDATEDIFFfunctions for calculating dates using date intervals You can use

them as a basis for calculating other dates that your applications might need

TIP

If you find yourself using any of these date calculations frequently, it might be a good

idea to create one or more user-defined functions to encapsulate these calculations It

would save your having to reenter the sometimes complex formulas, which can be

easi-ly mistyped, leading to incorrect calculations For information on creating user-defined

functions, see Chapter 29, “Creating and Managing User-Defined Functions.”

Converting Dates for Comparison

Because the datetimedata type contains both time and date components, searching for

data rows matching a specific date only, excluding the time component, can sometimes be

a bit tricky—especially when you consider that SQL Server stores time values only down to

3/1,000 second For example, if you want to find all rows where the date is for a certain

day, you have to perform a range search for all times within that day Because a date

without a time specified defaults to a time of midnight (00:00:00.000) for that date, the

following query doesn’t return all matching rows if any of the data values contain a time

other than midnight:

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select title_id, pubdate from dbo.titles where pubdate = ‘2006-01-14’

To be sure to include all rows for a particular date, regardless of the time component

stored, you could run a query similar to the following:

select title_id, pubdate from dbo.titles

where pubdate between ‘2006-01-14 00:00:00.0’ and ‘2006-01-14 23:59:59.997’

go

title_id pubdate

-

-FI3599 2006-01-14 00:00:00.000

Now you might be wondering, why use a time of ”2006-01-14 23:59:59.997”as the last

time of the day? You do so because SQL Server stores datetime values only down to

3/1,000 second If you enter a time of ”2006-01-14 23:59:59.999”, SQL Server rounds it

up to ”2006-01-15 00:00:00.000”, and it actually matches any rows with that datetime

value, as in this example:

title_id pubdate

-

-FI3599 2006-01-14 00:00:00.000

FI5162 2006-01-15 00:00:00.000

This is one reason you have to be careful when performing date searches Now you might

be wondering why not just use the DATEDIFFfunction as in the following example:

where datediff(day, pubdate, ‘2006-01-14’) = 0

go

title_id pubdate

-

-FI3599 2006-01-14 00:00:00.000

Although this query returns the correct result, the use of the function on the pubdate

column may prevent SQL Server from using any indexes that exist on the pubdatecolumn

to optimize the query, and it is likely to end up performing a table scan (For more

infor-mation on query optimization and optimizable search arguments, see Chapter 35.) To

help ensure that your queries are optimized effectively, you need to try to avoid using any

functions or expressions on the column in the search argument, and you need to search

against constant expressions

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Another way to write the preceding query would be to use the date calculations discussed

previously in this section For example, you could use the calculation to determine

midnight of the desired date and use that as the inclusive lower bound, and you could use

the calculation of midnight of the next day as the noninclusive upper bound and write a

query similar to the following:

declare @date datetime

set @date = ‘2006-01-14’

where pubdate >= DATEADD(dd, DATEDIFF(dd,’’,@date), ‘’)

and pubdate < DATEADD(dd, DATEDIFF(dd,’’,@date) + 1, ‘’)

go

title_id pubdate

-

-FI3599 2006-01-14 00:00:00.000

SQL Server 2008 introduces the dateandtimedata types, as well as the datetime2data

type The long-awaited dateandtimedata types store just a date value or time value,

respectively, making date-only or time-only comparisons much simpler For example, the

previous solution for finding all books published on a specific day can be simplified a bit

using the datedata type because there is no need to consider a time component:

declare @date date

set @date = ‘2006-01-14’

where pubdate >= @date

and pubdate < DATEADD(dd, 1, @date)

go

If thepubdatecolumn were defined using thedatedata type instead ofdatetime

(reason-able because the time of publication of a book is irrelevant), the comparison becomes

even simpler:

alter table titles drop constraint DF titles pubdate 103673A0

drop statistics titles.pubdate

alter table titles alter column pubdate date null

alter table titles

add constraint DF titles pubdate 103673A0

default getdate() for pubdate

go

declare @date date

set @date = ‘2006-01-14’

where pubdate = @date

go

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title_id pubdate

-

-FI3599 2006-01-14 00:00:00.000

Thedatetime2data type stores the time value down to microseconds and avoids the

3/1,000 second rounding issue that was present with the datetimedata type For example,

if you redefine the pubdatecolumn using the datetime2data type, you avoid the

round-ing issue and get a sround-ingle row as expected by the followround-ing query:

alter table titles drop constraint DF titles pubdate 103673A0

alter table titles alter column pubdate datetime2 null

alter table titles

add constraint DF titles pubdate 103673A0

default sysdatetime() for pubdate

go

title_id pubdate

-

-FI3599 2006-01-14 00:00:00.000

Sorting Results with the GROUPING Function

When working with the CUBEorROLLUPoperator, SQL Server generates NULLvalues for the

columns that are being rolled up to generate the aggregate values When you are viewing

the results, however, it can be difficult to determine whether the NULLvalue shown for a

nonaggregate column is the result of a rollup or because the column itself contains a NULL

value Fortunately, SQL Server provides the GROUPINGfunction, which you can use to

distinguish between real NULLvalues and NULLvalues that represent a rollup of all values

for a column in the result set

TheGROUPINGfunction returns 1when the value is grouped and 0when the column

contains a NULLvalue

In Listing 43.13, the GROUPINGfunction is used to replace NULLvalues for the rolled-up

columns with ALL

SELECT CASE when GROUPING(type) = 1 then ‘ALL’

else isnull(type, ‘Other’)

END AS type,

cast(CASE when (grouping(advance) = 1) then ‘ALL’

else isnull(convert(varchar(10), advance), ‘Unknown’)

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END as varchar(10)) as advance,

count(*) AS number

FROM DBO.titles

where type like ‘%cook%’ or type like ‘p%’

GROUP BY type, advance

WITH rollup

go

type advance number

- -

-mod_cook 0.00 1

mod_cook 15000.00 1

mod_cook ALL 2

popular_comp Unknown 1

popular_comp 7000.00 1

popular_comp ALL 3

psychology 2000.00 1

psychology ALL 5

trad_cook 4000.00 1

trad_cook 7000.00 1

trad_cook 8000.00 1

trad_cook ALL 3

ALL ALL 13

You can also use the GROUPINGfunction to order the result sets to move all the rollups

toward the bottom, as shown in Listing 43.14

SELECT CASE when GROUPING(type) = 1 then ‘ALL’

else isnull(type, ‘Other’)

END AS type,

cast(CASE when (grouping(advance) = 1) then ‘ALL’

else isnull(convert(varchar(10), advance), ‘Unknown’)

END as varchar(10)) as advance,

count(*) AS number

FROM DBO.titles

where type like ‘%cook%’ or type like ‘p%’

GROUP BY type, advance

WITH rollup

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ORDER by GROUPING(type), GROUPING(advance)

go

type advance number

- -

-popular_comp Unknown 1

trad_cook 4000.00 1

trad_cook 7000.00 1

trad_cook 8000.00 1

mod_cook 0.00 1

mod_cook 15000.00 1

mod_cook ALL 2

trad_cook ALL 3

psychology ALL 5

popular_comp ALL 3

ALL ALL 13

Using CONTEXT_INFO

Although SQL Server enables you to define local variables within a T-SQL batch or stored

procedure, local variables do not retain values between batches or stored procedures

Unfortunately, SQL Server 2008 does not enable you to create user-defined global

vari-ables However, you can simulate global variables by using the CONTEXT_INFOsetting,

which allows you to store information in the context_infocolumn in the

sys.sysprocessescatalog view A row in sys.sysprocessesexists for every connection to

SQL Server, so the data remains there until you disconnect from SQL Server

Thecontext_infocolumn is a binary (128)column You can store any data value in it

with the SET CONTEXT_INFOcommand, but you have to deal with hexadecimal data when

retrieving it If you are handy at manipulating hexadecimal data, you can store multiple

values in the context_infocolumn The following example stores the average price from

thetitlestable in the context_infocolumn:

declare @avg_price money

select @avg_price = avg(price) from dbo.titles

set context_info @avg_price

You can retrieve the value stored in context_infoby using a SELECTstatement You need

to convert the binarydata back to moneywhen you retrieve it Because avg(price)is the

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only value stored in context_info, you can retrieve it by performing a substring on the

first 8 bytes of the context_infocolumn (The moneydata type is 8 bytes in size.) Because

SQL Server assigns a unique server process ID (SPID) to each connection, you use the

@@SPIDfunction to retrieve the information for the current connection:

select convert(money, substring(context_info, 1, 8)) as AVG_PRICE

from master sysprocesses

where spid = @@spid

go

AVG_PRICE

-0.3751

If you don’t use a substring to specify only the first 8 bytes of thecontext_infocolumn,

SQL Server assumes that themoneydata is stored in the last 8 bytes and returns a result of0:

select convert(money, context_info) as AVG_PRICE

where spid = @@spid

go

AVG_PRICE

-0.00

Becausemoneycan be implicitly converted to binary, you don’t need to convert it when

settingcontext_info For some other data types, such as charordatetime, you need to

explicitly convert the data to binarybecause implicit conversions from those data types to

binaryis not supported In the following example, you append a datetimevalue to the

average price value already stored in context_info You explicitly convert the datetime

value to binaryand append it to the 8 bytes you have already stored in context_info:

declare @max_date datetime,

@context_info binary(128)

select @max_date = max(pubdate) from dbo.titles

select @context_info = substring(context_info, 1, 8)

+ convert(binary(8), @max_date) from master sysprocesses

where spid = @@spid

set context_info @context_info

You now have two values stored in context_info Using the appropriate substring, you

can retrieve either the average price or the maximum pubdate from context_info:

declare @avg_price money,

@max_pubdate datetime

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select @avg_price = substring(context_info, 1, 8),

@max_pubdate = substring(context_info, 9, 8)

where spid = @@spid

select @avg_price as ‘Avg Price’, @max_pubdate as ‘Max PubDate’

go

Avg Price Max PubDate

-

-0.3751 2009-05-31 00:00:00.000 Note that the binarydata converts implicitly to moneyanddatetime Working with Outer Joins An outer join is used to return all the rows from the specified outer table (specified with LEFT OUTER,RIGHT OUTER, or FULL OUTER), even if the other table has no match Rows returned from the outer table that have no corresponding match in the inner table display the value NULLfor any columns retrieved from the inner table For example, you might want to display the names of all authors along with the average royalty paid, if available: select au_lname, au_fname, avg(royaltyper) as avg_royalty from dbo.authors a left outer join dbo.titleauthor ta on a.au_id = ta.au_id group by au_lname, au_fname order by 3 go au_lname au_fname avg_royalty - -

-Greene Morningstar NULL Greenfield Tom NULL McBadden Heather NULL Smith Meander NULL Stringer Dirk NULL Gringlesby Burt 30

O’Leary Michael 35

Ringer Anne 37

Yokomoto Akiko 40

MacFeather Stearns 42

Hunter Sheryl 50

Dull Ann 50

Bennet Abraham 60

Green Marjorie 70

DeFrance Michel 75

Karsen Livia 75

Ringer Albert 75

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