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Generating unique sequences

Posted on 1998-12-02
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Last Modified: 2010-03-19
I am looking for a solution to the following problem (similar anyway):

There are banks, each banks having a BANK_NBR, and each bank opens bank accounts. When an new account is created, an account number is assigned to it. For a new account, a record is created in a table that has the following unique key: BANK_NBR, ACCOUNT_NBR.

My question relates to the case where several accounts are created at the same time in the same bank. How do I ensure that each one is assign a different number.  

I wish I could use an @@IDENTITY field but each bank needs an acount 1, 2, 3, 4...

The solution probably involves using a

SELECT MAX(ACCOUNT_NBR) + 1 from ACCOUNTS
WHERE BANK_NBR = PassedValue

I realize the chances are small that two accounts be created at the same time at the same bank (probably talking about milliseconds) but I need a robust solution.
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Question by:moonrises
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3 Comments
 
LVL 7

Accepted Solution

by:
tchalkov earned 400 total points
ID: 1092047
you can use the following construction:

begin transaction
SELECT MAX(ACCOUNT_NBR) + 1 from ACCOUNTS (tablockx)
WHERE BANK_NBR = PassedValue
.
//insert your new record here
.
commit tran
By using tablockx you require exclusive lock over the table so no one else could read from it until the end of the transaction.
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Author Comment

by:moonrises
ID: 1092048
Thank you for your answer. Just to confirm, if somebody tries to read the table while there is a lock, will it try for a while or will it return an error ?
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LVL 7

Expert Comment

by:tchalkov
ID: 1092049
it will wait until the lock is released
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