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Understanding Differences Between `Non-Repeatable Read` and `Phantom Read` in SQL

Disclaimer/Disclosure: Some of the content was synthetically produced using various Generative AI (artificial intelligence) tools; so, there may be inaccuracies or misleading information present in the video. Please consider this before relying on the content to make any decisions or take any actions etc. If you still have any concerns, please feel free to write them in a comment. Thank you. --- Summary: Explore the key differences between Non-Repeatable Read and Phantom Read in SQL, including their impact on database transactions and the concurrency issues they address. --- Understanding Differences Between Non-Repeatable Read and Phantom Read in SQL When dealing with database transactions and ensuring data integrity, understanding the different anomalies that can occur is crucial. Two common anomalies are Non-Repeatable Reads and Phantom Reads. This guide will break down these concepts and highlight their differences, helping you understand how they impact SQL transactions. Non-Repeatable Read A Non-Repeatable Read occurs when a transaction reads the same row multiple times and finds different data with each read. This typically happens when another transaction modifies the data between the reads. Non-Repeatable Reads can lead to inconsistencies within the same transaction since the data changes while the transaction is still in progress. Example Scenario for Non-Repeatable Read Transaction A reads a row from the database. Transaction B modifies and commits the changes to that row. Transaction A reads the same row again and finds different data. Phantom Read A Phantom Read involves a scenario where a transaction reads a set of rows that match a condition, but a subsequent read within the same transaction sees an additional row that matches the condition due to another transaction's insert or delete operations. This anomaly occurs when new rows that match the criteria of a query are added by another transaction between two reads of the first transaction. Example Scenario for Phantom Read Transaction X reads all rows where a specific condition (e.g., salary > 5000) is met. Transaction Y inserts a new row that matches this condition and commits. Transaction X reads the rows again and finds the newly inserted row, which wasn’t present during the initial read. Key Differences Nature of Anomaly Non-Repeatable Read: Deals with changes to existing rows that have already been read by a transaction. Phantom Read: Deals with the addition or deletion of rows that fit the criteria of an existing transaction’s query. Isolation Levels Different SQL isolation levels can help mitigate these anomalies: Repeatable Read: Prevents Non-Repeatable Reads by ensuring a transaction cannot modify a row once it's been read by another ongoing transaction. Serializable: Prevents Phantom Reads by ensuring that a transaction can view the database only as a static snapshot until it completes. Summary Understanding the differences between Non-Repeatable Read and Phantom Read is essential for managing transaction integrity and concurrency in SQL databases. Non-Repeatable Reads relate to changes in existing rows, while Phantom Reads involve the appearance or disappearance of rows meeting a certain condition. Utilizing appropriate isolation levels can help mitigate these anomalies and ensure robust database transaction handling. Whether you are optimizing a complex database application or learning about SQL transaction management, knowing the implications of these read anomalies is fundamental to your success.