How this works?

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How this works?

vignesh.ms

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Points: 3456
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July 24, 2014 at 8:11 am

#293692

Hi there ,
I got this code from web, I cant understand how it works
can anybody suggest me with links or explanations about the below used techniques.
DECLARE @t TABLE( ID INT IDENTITY, data VARCHAR(20))
INSERT INTO @t(data) SELECT 'Jacob'
INSERT INTO @t(data) SELECT 'Sebastian'
;WITH Numbers(N) AS (
SELECT DISTINCT Number
FROM master..spt_values
WHERE Number BETWEEN 1 AND (SELECT MAX(LEN(data))FROM @t)
)
SELECT
id,
data_reversed
FROM @t T1
CROSS APPLY(
SELECT
SUBSTRING(data,N,1)
FROM @t T2 JOIN Numbers ON N<=LEN(data)
WHERE T1.id=T2.id
ORDER BY N DESC
FOR XML PATH('')
)x(data_reversed)
ORDER BY id DESC

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ChrisM@Work SSC Guru Points: 186127 More actions · Answer 1

-- make a table with two rows, two columns

DECLARE @t TABLE( ID INT IDENTITY, data VARCHAR(20))

INSERT INTO @t(data) SELECT 'Jacob'

INSERT INTO @t(data) SELECT 'Sebastian'

SELECT * FROM @t

GO

----------------------------------------------------------------------------------------------------

DECLARE @t TABLE( ID INT IDENTITY, data VARCHAR(20))

INSERT INTO @t(data) SELECT 'Jacob'

INSERT INTO @t(data) SELECT 'Sebastian'

-- a CTE which provides n rows containing values 1 through n,

-- where n is the longest word in column [data]

SELECT DISTINCT Number

FROM master..spt_values

WHERE Number BETWEEN 1 AND (SELECT MAX(LEN(data))FROM @t)

GO

----------------------------------------------------------------------------------------------------

DECLARE @t TABLE( ID INT IDENTITY, data VARCHAR(20))

INSERT INTO @t(data) SELECT 'Jacob'

INSERT INTO @t(data) SELECT 'Sebastian'

-- pick the letters from [data] one by one, putting each one on a separate row of the output

;WITH Numbers(N) AS (

SELECT DISTINCT Number

FROM master..spt_values

WHERE Number BETWEEN 1 AND (SELECT MAX(LEN(data))FROM @t)

)

SELECT T2.id, N,

SUBSTRING(data,N,1)

FROM @t T2 JOIN Numbers ON N<=LEN(data)

--WHERE T1.id=T2.id

ORDER BY T2.id, N DESC -- added T2.id for clarity

--FOR XML PATH('')

GO

----------------------------------------------------------------------------------------------------

-- finally, FOR XML PATH concatenates the values from each row (for each id)

-- ORDER BY N DESC ensures that the letters are concatenated in the reverse order from collection.

DECLARE @t TABLE( ID INT IDENTITY, data VARCHAR(20))

INSERT INTO @t(data) SELECT 'Jacob'

INSERT INTO @t(data) SELECT 'Sebastian'

;WITH Numbers(N) AS (

SELECT DISTINCT Number

FROM master..spt_values

WHERE Number BETWEEN 1 AND (SELECT MAX(LEN(data))FROM @t)

)

SELECT

id,

data_reversed

FROM @t T1

CROSS APPLY(

SELECT

SUBSTRING(data,N,1)

FROM @t T2 JOIN Numbers ON N<=LEN(data)

WHERE T1.id=T2.id

ORDER BY N DESC

FOR XML PATH('')

)x(data_reversed)

ORDER BY id DESC

----------------------------------------------------------------------------------------------------

^{“Write the query the simplest way. If through testing it becomes clear that the performance is inadequate, consider alternative query forms.” - Gail Shaw}

For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden

ChrisM@Work SSC Guru Points: 186127 More actions · Answer 2

vignesh.ms (7/24/2014)
Hi there ,
I got this code from web, I cant understand how it works
can anybody suggest me with links or explanations about the below used techniques.
.
.
.

An excellent article here[/url].

^{“Write the query the simplest way. If through testing it becomes clear that the performance is inadequate, consider alternative query forms.” - Gail Shaw}

For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden

Luis Cazares SSC Guru Points: 183706 More actions · Answer 3

I hope that you're using that code for learning purposes only and not instead of REVERSE() function.

Luis C.
General Disclaimer:
Are you seriously taking the advice and code from someone from the internet without testing it? Do you at least understand it? Or can it easily kill your server?

How to post data/code on a forum to get the best help: Option 1 / Option 2

vignesh.ms Hall of Fame Points: 3456 More actions · Answer 4

Luis Cazares (7/24/2014)
I hope that you're using that code for learning purposes only and not instead of REVERSE() function.

yes of course

vignesh.ms Hall of Fame Points: 3456 More actions · Answer 5

ChrisM@Work (7/24/2014)
-- make a table with two rows, two columns DECLARE @t TABLE( ID INT IDENTITY, data VARCHAR(20)) INSERT INTO @t(data) SELECT 'Jacob' INSERT INTO @t(data) SELECT 'Sebastian' SELECT * FROM @t GO ---------------------------------------------------------------------------------------------------- DECLARE @t TABLE( ID INT IDENTITY, data VARCHAR(20)) INSERT INTO @t(data) SELECT 'Jacob' INSERT INTO @t(data) SELECT 'Sebastian' -- a CTE which provides n rows containing values 1 through n, -- where n is the longest word in column [data] SELECT DISTINCT Number FROM master..spt_values WHERE Number BETWEEN 1 AND (SELECT MAX(LEN(data))FROM @t) GO ---------------------------------------------------------------------------------------------------- DECLARE @t TABLE( ID INT IDENTITY, data VARCHAR(20)) INSERT INTO @t(data) SELECT 'Jacob' INSERT INTO @t(data) SELECT 'Sebastian' -- pick the letters from [data] one by one, putting each one on a separate row of the output ;WITH Numbers(N) AS ( SELECT DISTINCT Number FROM master..spt_values WHERE Number BETWEEN 1 AND (SELECT MAX(LEN(data))FROM @t) ) SELECT T2.id, N, SUBSTRING(data,N,1) FROM @t T2 JOIN Numbers ON N<=LEN(data) --WHERE T1.id=T2.id ORDER BY T2.id, N DESC -- added T2.id for clarity --FOR XML PATH('') GO ---------------------------------------------------------------------------------------------------- -- finally, FOR XML PATH concatenates the values from each row (for each id) -- ORDER BY N DESC ensures that the letters are concatenated in the reverse order from collection. DECLARE @t TABLE( ID INT IDENTITY, data VARCHAR(20)) INSERT INTO @t(data) SELECT 'Jacob' INSERT INTO @t(data) SELECT 'Sebastian' ;WITH Numbers(N) AS ( SELECT DISTINCT Number FROM master..spt_values WHERE Number BETWEEN 1 AND (SELECT MAX(LEN(data))FROM @t) ) SELECT id, data_reversed FROM @t T1 CROSS APPLY( SELECT SUBSTRING(data,N,1) FROM @t T2 JOIN Numbers ON N<=LEN(data) WHERE T1.id=T2.id ORDER BY N DESC FOR XML PATH('') )x(data_reversed) ORDER BY id DESC
----------------------------------------------------------------------------------------------------

Good explanation...

Splitting query into chunks make me understand easily. Really its very useful to me. Thanks for your time & effort..