Скачать или смотреть Using ROW_NUMBER() with Filtering in TSQL

Discover how to effectively use the `ROW_NUMBER()` function with filtering in TSQL to achieve desired results. Learn tips and techniques for optimizing your SQL queries.
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How to Use Filtering in ROW_NUMBER() TSQL

When working with SQL Server, one of the most powerful functions at your disposal is ROW_NUMBER(). This function allows you to assign a unique sequential integer to rows within a partition of a result set. However, when working with specific conditions, such as filtering based on a column’s value, it can become a bit tricky. In this guide, we'll discuss how to effectively implement filtering with ROW_NUMBER() in TSQL.

The Problem

Imagine you have a database table that tracks the status of tasks assigned to various individuals. Here’s a simplified view of your table structure:

SNNameStatus1SamOnHold2SamOnHold3SamNotOnHold4SamOnHold5AlanOnHold6AlanNotOnHold7AlanOnHold8ElisNotOnHold9ElisNotOnHold10ElisNotOnholdYour objective is to generate a result set that includes a rank for the tasks that are currently “OnHold”, grouped by the name of the person. Here’s how you’d like your output to look:

SNNameStatusRank1SamOnHold12SamOnHold23SamNotOnHoldNULL4SamOnHold35AlanOnHold16AlanNotOnHoldNULL7AlanOnHold28ElisNotOnHoldNULL9ElisNotOnHoldNULL10ElisNotOnHoldNULLWith your current approach using UNION ALL, you’ve achieved the desired output, but it may not be the most optimized solution.

The Proposed Solution

Using CASE with ROW_NUMBER()

Instead of relying on unions, you can simplify your TSQL by applying a CASE statement within the query. This allows you to evaluate the status within a single query without needing to combine multiple select statements. Here’s how to do it:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Query:

The SELECT statement retrieves columns SN, name, and status from the table.

The CASE statement checks the status of each row.

If the status is 'OnHold', the ROW_NUMBER() function is applied, partitioning by name and ordering by SN, effectively assigning a rank.

If the status is not 'OnHold', it returns NULL for the rank.

The result set is ordered by SN to maintain a structured output.

Benefits of This Method:

Efficiency: This method eliminates the need for multiple queries and the overhead of union operations.

Simplicity: The logic is encapsulated within a single query, making it easier to read and maintain.

Flexibility: You can easily adapt the conditions in the CASE statement to incorporate more complex scenarios.

Conclusion

Utilizing the ROW_NUMBER() function in TSQL when filtering data based on a specific condition can significantly streamline your queries. By applying a CASE expression, not only do you get the desired rankings, but you also improve the efficiency and readability of your SQL queries.

Whether you’re working on a personal project or managing production databases, mastering TSQL functions like ROW_NUMBER() will elevate your SQL skills. Happy querying!