Isolation levels are an important concept in SQL Server that help protect resources and ensure data integrity in a multi-user environment. By setting the appropriate isolation level, you can control how transactions interact with each other and prevent concurrency-related issues.

There are five isolation levels defined in SQL Server, ranging from the lowest level to the highest level:

1. Read Uncommitted
2. Read Committed
3. Repeatable Read
4. Serializable
5. Snapshot

Let’s take a closer look at each isolation level and understand their characteristics:

1. Read Uncommitted

This is the least restrictive isolation level. In Read Uncommitted, a transaction can read data that has been modified by other transactions but not yet committed. This level does not issue shared locks to prevent data modification and may result in problems like dirty reads, lost updates, and phantom reads.

2. Read Committed

Read Committed is the default isolation level set by SQL Server for any database. It prevents transactions from reading data that has been modified by other transactions but not yet committed. This level eliminates the problem of dirty reads but does not eliminate the occurrence of phantom reads and repeatable reads.

3. Repeatable Read

Repeatable Read is a higher isolation level that does not allow any transaction to read data that is being modified by other transactions but not yet committed. It also prevents any transaction from modifying data that is being read by other transactions until the reading transaction completes its operation. This level eliminates the problems of dirty reads and repeatable reads but does not eliminate phantom reads.

4. Serializable

Serializable is a strict isolation level that does not allow any transaction to read data until other transactions have completed their data modification operations. It also does not allow other transactions to modify data until the current transaction has completed its read operation. This level ensures that a transaction acquires a read or write lock for the entire range of records it is going to affect. It eliminates the problem of phantom reads.

5. Snapshot

In Snapshot isolation level, a transaction only recognizes data that was committed before the start of the transaction. Any modifications made to the data after the transaction has begun are not visible to the currently executing transaction. This level creates a snapshot of the data for each transaction using row versioning. It eliminates problems like dirty reads, lost updates, repeatable reads, and phantom reads.

You can set the isolation level using the following command:

SET TRANSACTION ISOLATION LEVEL [IsolationLevel]

Understanding and choosing the right isolation level for your database is crucial to ensure data consistency and prevent concurrency-related issues. Each isolation level has its own trade-offs, and it’s important to consider the specific requirements of your application.

By carefully selecting the appropriate isolation level, you can strike a balance between concurrency and data integrity in your SQL Server environment.