Sql updating a row
Modifications made by concurrent transactions must be isolated from the modifications made by any other concurrent transactions.A transaction either recognizes data in the state it was in before another concurrent transaction modified it, or it recognizes the data after the second transaction has completed, but it does not recognize an intermediate state.
All internal data structures, such as B-tree indexes or doubly-linked lists, must be correct at the end of the transaction.
When a transaction is started on a connection, all Transact-SQL statements executed on that connection are part of the transaction until the transaction ends.
However, under a multiple active result set (MARS) session, a Transact-SQL explicit or implicit transaction becomes a batch-scoped transaction that is managed at the batch level.
So while updating the TARGET table, TERADATA won’t be able to understand from which row it should update the TARGET row i.e.
whether the CUST_CITY of TARGET table should be updated by DELHI or by CHENNAI. You need to make sure that there is only one row in SOURCE table while updating the TARGET.
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This guide describes the locking and row versioning mechanisms the SQL Server Database Engine uses to ensure the physical integrity of each transaction and provides information on how applications can control transactions efficiently.