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Discover effective SQL techniques to retrieve a limited number of records from multiple department categories in a single query.
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How to Filter Specific Number of Records from Each Value in SQL Clause

When working with large datasets in SQL, particularly in situations where we need to manage records spread across multiple categories—like employees in various departments—the challenge often arises in how to filter these records effectively.

Imagine having an employee database with 100,000 records representing employees from different departments, and you want to extract a specific number of records from each department. This scenario can be even trickier when you're using an SQL IN clause to target multiple departments at once.

The Problem

You have an employee table structured as follows:

EmpNumber: Unique identifier for each employee

EmpName: Name of the employee

EmpAge: Age of the employee

DepartmentNumber: Identifier for the department

DepartmentName: Name of the department

Suppose you aim to filter only 10 employee records from each specified department, say 'xxx' and 'yyyy'. The limitation with a basic query like this:

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

Is that it only returns a maximum of 10 records from the first department it matches, ignoring any records from the second department if it also has more than 10 records.

The Solution

To tackle this issue, we leverage the row_number() SQL function. This function allows us to assign a unique sequential integer to rows within a partition of a result set.

Step-by-Step Query Breakdown

Here's how to structure your query using row_number():

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

Explanation of the Query

Subquery Creation:

Inside the parentheses, we are selecting all columns from the employee table (SELECT e.*).

We also create a new column (seqnum) using the ROW_NUMBER() function.

Partitioning:

The PARTITION BY DepartmentNumber directive groups the results by department.

This means ROW_NUMBER() will reset for each distinct DepartmentNumber, allowing us to number records from the same department separately.

Ordering:

Depending on your needs, you can order by another column (e.g., ORDER BY EmpName if you want the employees sorted alphabetically).

Filtering Results:

Finally, in the outer query, we filter based on seqnum. Here, we only want rows where seqnum is less than or equal to 10—resulting in up to 10 employee records from each specified department.

Conclusion

By using the row_number() function combined with partitioning, you can efficiently retrieve a limited number of records from each distinct category, such as departments in your employee table. This method not only addresses the filtering issue but also enhances the flexibility of your SQL queries, allowing for better data management in large databases.

With this approach, you can confidently handle similar scenarios in your database queries, ensuring that you get a balanced and representative sample of data from each category required.

Whether you're a seasoned SQL developer or a beginner, mastering this technique will significantly improve your data querying capabilities. Happy querying!