Understanding Why the SQL INSERT INTO SELECT Statement Can Duplicate Rows and How to Fix It скачать в хорошем качестве

Understanding Why the SQL INSERT INTO SELECT Statement Can Duplicate Rows and How to Fix It

Discover the common issue of row duplication in SQL `INSERT INTO SELECT` statements and learn how to modify your stored procedure to save only unique results. --- This video is based on the question https://stackoverflow.com/q/72478806/ asked by the user 'MarwanAbu' ( https://stackoverflow.com/u/18314179/ ) and on the answer https://stackoverflow.com/a/72479710/ provided by the user 'Hogan' ( https://stackoverflow.com/u/215752/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions. Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Why SQL INSERT INTO SELECT statement duplicates the rows? Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l... The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license. If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com. --- Understanding Why the SQL INSERT INTO SELECT Statement Can Duplicate Rows and How to Fix It When working with SQL databases, you may encounter situations where executing an INSERT INTO SELECT statement leads to duplicated rows being inserted. This can be especially frustrating when your goal is to maintain data integrity and ensure that only unique entries are recorded. In this guide, we will explore a common scenario involving C- and SQL Server that illustrates this issue, and provide a detailed solution to prevent duplications in your database. The Problem: Duplicate Rows in SQL INSERT Statements Suppose you are developing a C- Windows application that utilizes a stored procedure to insert results from one table into another. You want to ensure that only one row per entry, specifically the old result, is saved in the target table. However, after executing your stored procedure, you notice that each row is duplicated twice. Example of Duplicating Rows Here’s a simplified version of your stored procedure that illustrates the problem: [[See Video to Reveal this Text or Code Snippet]] In this setup, you are inserting from the LAB_RESULTS table into the LAB_RESULTS_UPDATED table. However, the use of EXCEPT may allow for unintended duplicates if certain conditions aren’t met. The Solution: Modifying the Stored Procedure To address the issue of duplicate rows, we can refine the SQL INSERT INTO SELECT statement. The trick is to use a LEFT JOIN instead of relying on EXCEPT. A LEFT JOIN allows us to filter for only those records that aren’t already present in the LAB_RESULTS_UPDATED table. Revised Stored Procedure Here is an updated version of your stored procedure: [[See Video to Reveal this Text or Code Snippet]] Explanation of the Changes LEFT JOIN: By joining the LAB_RESULTS table with an inner query that selects existing entries in the LAB_RESULTS_UPDATED table, we can effectively filter out already inserted rows. WHERE Clause: The condition AND EX.ORDER_ID IS NULL ensures that only unique records that do not exist in the LAB_RESULTS_UPDATED table are inserted. Conclusion In summary, the SQL INSERT INTO SELECT statement can result in duplication if conditions are not properly set up. By modifying your stored procedure to utilize a LEFT JOIN instead of EXCEPT, you can ensure that only unique records are inserted into your target table. This streamlined approach helps maintain data integrity and enhances the reliability of your application. Implementing these changes should help you resolve the issue of duplicated rows in your SQL applications. If you have further questions or encounter other issues, feel free to ask for assistance!