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Changing Identity Columns


Changing Identity Columns

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kapil_kk
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Execute this code and you will get duplicate values in Identity values--

CREATE TABLE IdentTest
( Ident INT NOT NULL IDENTITY (1,1)
, varfield varchar(100)
)

INSERT INTO IdentTest VALUES ('xyz') -- <== Added to original
INSERT INTO IdentTest VALUES ('123') -- <== Added to original

SELECT Ident FROM IdentTest
-- Result 1,2

DBCC CHECKIDENT ('IdentTest',RESEED,100)
--Checking identity information: current identity value '2', current column value '100'.

INSERT INTO IdentTest VALUES ('abc')

SELECT Ident FROM IdentTest
--Result 1,2,101

DBCC CHECKIDENT ('IdentTest',RESEED,100)
--Checking identity information: current identity value '101', current column value '100'.

INSERT INTO IdentTest VALUES ('abc')

SELECT Ident FROM IdentTest
--Result 1,2,101,101

Its intresting to know that functionality of identity is exploiting in this way

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Lokesh Vij
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kapil_kk (10/22/2012)
Execute this code and you will get duplicate values in Identity values--

CREATE TABLE IdentTest
( Ident INT NOT NULL IDENTITY (1,1)
, varfield varchar(100)
)

INSERT INTO IdentTest VALUES ('xyz') -- <== Added to original
INSERT INTO IdentTest VALUES ('123') -- <== Added to original

SELECT Ident FROM IdentTest
-- Result 1,2

DBCC CHECKIDENT ('IdentTest',RESEED,100)
--Checking identity information: current identity value '2', current column value '100'.

INSERT INTO IdentTest VALUES ('abc')

SELECT Ident FROM IdentTest
--Result 1,2,101

DBCC CHECKIDENT ('IdentTest',RESEED,100)
--Checking identity information: current identity value '101', current column value '100'.

INSERT INTO IdentTest VALUES ('abc')

SELECT Ident FROM IdentTest
--Result 1,2,101,101

Its intresting to know that functionality of identity is exploiting in this way


Very true..that's what I was trying to point out:-)

~ Lokesh Vij

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Koen Verbeeck
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Lokesh Vij (10/22/2012)
kapil_kk (10/22/2012)
in this case the max ident value would be 101 as with first insert statement the value of ident was 100 and with another insert statement the ident value would be 101 and max from 101 and 100 is 101 so 101 is the max value..
hope its clear to you


Actually the question posted by John is little different. Execute the below code:

CREATE TABLE IdentTest 
( Ident INT NOT NULL IDENTITY (1,1)
, varfield varchar(100)
)

INSERT INTO IdentTest VALUES ('xyz') -- <== Added to original
INSERT INTO IdentTest VALUES ('123') -- <== Added to original

DBCC CHECKIDENT ('IdentTest',RESEED,100)

INSERT INTO IdentTest VALUES ('abc')

SELECT Ident FROM IdentTest -- <== (done away with max)



Now the last select statement (done away with max statement intentionally), returns three values 1,2 and 101
Why value 101?? Why not 100 or 102??


If data is present in the table before DBCC CHECKIDENT, the new identity value is the reseed value + current incremenet. In this case, 100 + 1 = 101.

Anyway, nice question.


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paul s-306273
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Nice question.
Stuart Davies
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Good question and thanks for an easy start to the week.

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john.arnott
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Koen Verbeeck (10/22/2012)
Lokesh Vij (10/22/2012)
kapil_kk (10/22/2012)
in this case the max ident value would be 101 as with first insert statement the value of ident was 100 and with another insert statement the ident value would be 101 and max from 101 and 100 is 101 so 101 is the max value..
hope its clear to you


Actually the question posted by John is little different. Execute the below code:

CREATE TABLE IdentTest 
( Ident INT NOT NULL IDENTITY (1,1)
, varfield varchar(100)
)

INSERT INTO IdentTest VALUES ('xyz') -- <== Added to original
INSERT INTO IdentTest VALUES ('123') -- <== Added to original

DBCC CHECKIDENT ('IdentTest',RESEED,100)

INSERT INTO IdentTest VALUES ('abc')

SELECT Ident FROM IdentTest -- <== (done away with max)



Now the last select statement (done away with max statement intentionally), returns three values 1,2 and 101
Why value 101?? Why not 100 or 102??


If data is present in the table before DBCC CHECKIDENT, the new identity value is the reseed value + current incremenet. In this case, 100 + 1 = 101.

Anyway, nice question.

Thank you Koen for the good succinct way to state how this works. It did tickle my urge to keep playing, though...
Try predicting the result of the final SELECT in this script:
CREATE TABLE IdentTest 
( --Ident INT NOT NULL IDENTITY (1,1)
varfield varchar(100)
)

INSERT INTO IdentTest VALUES ('xyz')
INSERT INTO IdentTest VALUES ('123')

Select * from IdentTest

ALTER TABLE IdentTest
Add Ident INT IDENTITY (1,1)

INSERT INTO IdentTest VALUES ('abc')

SELECT Ident FROM IdentTest
where varfield = 'abc'


Stewart "Arturius" Campbell
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Good back-to-basics question, thanks, Kenneth

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TomThomson
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Nice easy question.

Tom

Iulian -207023
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Nice one, thanks

Iulian
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Good question, Thanks. Also, enjoy the discussion that followed.
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