What's the Key Difference Between SQL Server's Read Committed and Repeatable Read Isolation Levels?

SQL Server Isolation Levels: Read Committed and Repeatable Read – A Detailed Comparison

SQL Server offers various isolation levels to manage data consistency during concurrent transactions. "Read committed" and "repeatable read" are two such levels, each providing different guarantees. This article clarifies their key distinctions.

Read Committed Isolation Level

With "read committed," any data retrieved is guaranteed to have been successfully committed to the database. This prevents reading uncommitted, or "dirty," data that might be rolled back later. However, a crucial limitation is that data read within a transaction is not guaranteed to remain unchanged. Concurrent transactions can modify or delete the data, and subsequent reads within the same transaction will reflect these changes.

Repeatable Read Isolation Level

"Repeatable read" offers stronger isolation than "read committed." Besides ensuring that all read data is committed, it further guarantees that data read within a transaction remains unchanged by other transactions. Subsequent reads of the same data will consistently return the identical results. This is critical when data consistency throughout the transaction is paramount, such as in financial transactions or data analysis.

Illustrative Example:

Imagine a "Customers" table with a "Balance" column initially set to 100.

1. Transaction A begins and reads the "Balance."
2. Transaction B concurrently withdraws 20 and commits.
3. Under "read committed," Transaction A's subsequent read will show the updated balance of 80.
4. Under "repeatable read," Transaction A's subsequent read will still display the original balance of 100, maintaining data consistency within the transaction.

Choosing the Right Isolation Level

Selecting the appropriate isolation level is vital for database application integrity. "Read committed" suits applications with infrequent, non-critical data updates. "Repeatable read" is essential when data integrity is paramount, ensuring reliable and accurate database operations. The choice depends on the specific application's requirements for data consistency and concurrency.

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