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Question :
What is a transaction isolation level and What is the advantage of it? What are different options that we set for a transaction isolation level? Transaction isolation levels are used when we go for transactions in stored procedures used in Transact SQL.
Category SQL Server Interview Questions
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Added on 4/3/2013
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Source: CoolInterview.com

Transaction Lavel isolation:- The isolation level used during the execution of SQL statements determines the degree to which the activation group is isolated from concurrently executing activation groups.

Thus, when activation group P executes an SQL statement, the isolation level determines:

* The degree to which rows retrieved by P and database changes made by P are available to other concurrently executing activation groups.
* The degree to which database changes made by concurrently executing activation groups can affect P.

There are many advantages to this approach: read-intensive applications typically want more index structures, data redundancies, and even other views of data. Transaction processing systems want the best write throughput while incurring only the most minimal overhead. The access patterns of readers and writers typically differ: Readers are more prone to larger analysis types of queries, and writers are more prone to singleton inserts, updates, and deletes. When these activities are separated, the administrator can focus on recovery strategies for a smaller, more manageable transaction processing system. OLTP databases tend to be much smaller than data redundant decision-support or analysis-oriented databases.

There are four isolation levels:

1. READ UNCOMMITTED
2. READ COMMITTED
3. REPEATABLE READ
4. SERIALIZABLE

1. Read uncommitted
When it's used, SQL Server not issue shared locks while reading data. So, you can read an uncommitted transaction that might get rolled back later. This isolation level is also called dirty read. This is the lowest isolation level. It ensures only that a physically corrupt data will not be read.

2.Read committed
This is the default isolation level in SQL Server. When it's used, SQL Server will use shared locks while reading data. It ensures that a physically corrupt data will not be read and will never read data that another application has changed and not yet committed, but it does not ensure that the data will not be changed before the end of the transaction.

3.Repeatable read
When it's used, the dirty reads and nonrepeatable reads cannot occur. It means that locks will be placed on all data that is used in a query, and another transactions cannot update the data.

4.Serializable
Most restrictive isolation level. When it's used, then phantom values cannot occur. It prevents other users from updating or inserting rows into the data set until the transaction is complete.
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