TRANSACTION | SQL Server Interview Questions

1. What is Transaction?
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 ACID (Atomicity, Consistency, Isolation, and Durability) 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 sees data in the state it was in before another concurrent transaction modified it, or it sees the data after the second transaction has completed, but it does not see 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.

2. After one Begin Transaction a truncate statement and a RollBack statements are there. Will it be rollbacked? Since the truncate statement does not perform logged operation how does it RollBack?
It will rollback.
 Given a SQL like
Begin Tran
   Select @@Rowcount
Begin Tran
   Select @@Rowcount
Begin Tran
   Select @@Rowcount
Commit Tran
   Select @@Rowcount
RollBack
   Select @@Rowcount
RollBack
   Select @@Rowcount
What is the value of @@Rowcount at each stmt levels?
Ans : 0 – zero.
@@ROWCOUNT - Returns the number of rows affected by the last statement.
@@TRANCOUNT - Returns the number of active transactions for the current connection.
Each Begin Tran will add count, each commit will reduce count and ONE rollback will make it 0.