January 4, 2008 at 12:22 am
Please, Give Me Answer in Brief
January 4, 2008 at 12:36 am
So that changes made by one user don't affect another concurrent query (automatically done by SQL Server)
To ensure that a related set of data modifications are completed entirely or not at all if the server fails or the connection drops
So that changes can be undone in case of an error.
Books online
A transaction is a sequence of operations performed as a single logical unit of work. A logical unit of work must exhibit four properties, called the atomicity, consistency, isolation, and durability (ACID) properties, to qualify as a transaction.
Atomicity
A transaction must be an atomic unit of work; either all of its data modifications are performed, or none of them is performed.
Consistency
When completed, a transaction must leave all data in a consistent state. In a relational database, all rules must be applied to the transaction's modifications to maintain all data integrity. All internal data structures, such as B-tree indexes or doubly-linked lists, must be correct at the end of the transaction.
Isolation
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. This is referred to as serializability because it results in the ability to reload the starting data and replay a series of transactions to end up with the data in the same state it was in after the original transactions were performed.
Durability
After a transaction has completed, its effects are permanently in place in the system. The modifications persist even in the event of a system failure.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability
January 4, 2008 at 6:02 am
Because we like our data
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"The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood"
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SQL Server Execution Plans
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