Скачать или смотреть How to Select Rows Where seqnum is a Specific Value in SQL

Learn the steps to filter SQL query results to only include rows with a specific `seqnum` value, ensuring unique `LongName` entries.
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Filtering SQL Query Results by seqnum

When working with SQL queries, especially in SQL Server, you might often find yourself needing to refine your results based on specific conditions. In this post, we're going to explore how to select only the rows where the seqnum column has a specific value—in this case, where seqnum = 1.

The Problem

You have a SQL query that fetches various fields from two joined tables, but the results include all rows rather than only those with seqnum equal to one. As shown in your query output, you want to only include rows where LongName is unique and seqnum is specifically set to 1.

Understanding the Query Structure

Here’s a breakdown of your current SQL query:

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

You're using the DENSE_RANK() function which ranks rows within a partition of a result set.

The PARTITION BY [LongName] means that the ranking resets for each unique LongName.

The Solution

While you cannot simply apply a WHERE clause directly on seqnum (as it is computed in the same SELECT statement), you can solve this problem using a Common Table Expression (CTE).

Step 1: Implement a CTE

A CTE allows you to temporarily define a result set that can be referred to within a SELECT, INSERT, UPDATE, or DELETE statement. Here’s how you can structure your modified query:

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

Step 2: Explanation of the CTE Structure

CTE Declaration:

The CTE is defined with the keyword WITH, followed by the name CTE.

Within parentheses, the SELECT statement calculates seqnum.

Main Query:

After the CTE, you have a main query that selects all records from the CTE.

You filter these records specifically where seqnum equals 1.

Ordering Results:

The result set is ordered by [Time] in descending order, remaining true to your original requirements.

Results

With this structured CTE approach, the filtered output will now include only those rows where seqnum equals 1. You will see a concise result set that meets your requirements, just as desired:

InstanceIDNumberValueLongNameEmployeeTimeseqnum4015105.993Parameter C105:10.012015117.298Parameter D105:10.011215126.925Parameter E005:10.014615132.83Parameter B405:10.014531GG2Parameter A005:10.01Conclusion

Using a CTE in your SQL query allows for more complex filtering based on calculated columns like seqnum. This method provides clarity and efficiency when querying for specific datasets, ensuring you can focus on the unique rows that matter. Implement this method in your projects to streamline your data retrieval processes effectively.

Remember to test your queries and ensure they provide the desired outcomes in your data context!