How to Return Previous and Next Value in Sequence Using SQL Server

Описание: Learn how to efficiently retrieve previous and next values in a sequence from your SQL Server database, utilizing proper aliases for accurate results.
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This video is based on the question https://stackoverflow.com/q/68355260/ asked by the user 'Dizzy49' ( https://stackoverflow.com/u/836924/ ) and on the answer https://stackoverflow.com/a/68355305/ provided by the user 'Squirrel' ( https://stackoverflow.com/u/185647/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the Problem: Fetching Previous and Next Values in a Sequence

In database management, one common requirement is the ability to retrieve not only the current value from a sequence but also to find the previous and next values based on that current value. This can be particularly useful in scenarios where tasks or items need to be processed in a specific order.

Imagine you have a table that stores tasks with a sequence assigned to each. Your goal is to structure the data to include the previous, current, and next task in the sequence for better visibility and tracking.

The Table Structure

Here's an overview of the sample table we are dealing with, which comprises task-related information alongside their respective sequences:

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The Objective

The desired output from this table is structured as follows:

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The Initial Query

The original SQL query designed to achieve this result was as follows:

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The Issue Identified

Upon executing this query, the results did not return the expected previous and next values, leaving us with unexpected NULL values. The main reason for this is that the column references in the subqueries weren't explicitly defined with table aliases.

Key Points:

Alias Usage: Without properly specifying the source of the Seq column in the subqueries, SQL Server may assume it is from the t1 alias.

Casting Confusion: The casting to int is correct, but the ambiguity in column sourcing causes issues.

The Solution

Fixing the Query

To rectify the situation, we must ensure that the second reference to the column is properly aliased as follows:

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A More Efficient Approach

Using LEAD() and LAG() functions can simplify the query and avoid subqueries altogether. For instance:

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Conclusion

Retrieving previous and next values from a sequence in SQL Server can be tricky if column aliases are not handled properly. By using proper aliases and potentially leveraging window functions like LEAD() and LAG(), you can create a more efficient and effective query. This not only improves readability but also enhances performance and accuracy in your results.

With these adjustments, your SQL queries will yield the expected outputs, providing clear insights into your task sequences!