How to Write an SQL Select Query to Retrieve Active Employees from Multiple Tables

Description:
Discover how to correctly structure your SQL Select query to get the latest active status of employees from Employee and EmployeeHistory tables.
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This video is based on the question https://stackoverflow.com/q/75834126/ asked by the user 'Gulfam' ( https://stackoverflow.com/u/1389411/ ) and on the answer https://stackoverflow.com/a/75834822/ provided by the user 'Joel Coehoorn' ( https://stackoverflow.com/u/3043/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Write an SQL Select Query to Retrieve Active Employees from Multiple Tables

When working with databases, it’s common to encounter situations where you need to extract specific data based on certain conditions. One such scenario is selecting employees whose latest status is "Active" from multiple related tables. In this guide, we’ll explore how to effectively write an SQL query to fulfill this requirement.

The Problem: Fetching Latest Active Employee Status

Consider a database with two tables: Employee and EmployeeHistory, which store employee details and their respective employment status, respectively. The challenge here is to select only those employees whose latest status is Active in the EmployeeHistory table.

Sample Database Structure

Here’s a simplified view of what the tables look like:

Employee Table:

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

EmployeeHistory Table:

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

From this data, we want to extract only John and Allan as they are the ones with the latest active status in the EmployeeHistory table.

Common SQL Query Attempt

The original SQL query posed issues, as it might not accurately reflect the desired outcomes. Here’s the query that was initially presented:

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

Why the Original Query Doesn’t Work

Subquery Complexity: The subquery can get confusing and might not efficiently return the latest status for each employee.

Possibility of Error: The use of TOP 1 does not guarantee that it will work as intended if there are multiple records per employee.

A Better Solution

Let’s improve the query to achieve the desired results effectively. The corrected SQL query is given below:

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

Breakdown of the Improved Query

Here’s how this query works:

Subquery with ROW_NUMBER():

We create a subquery using ROW_NUMBER() which partitions the employee records by EmployeeID and orders them by ID in descending order.

This way, we can identify the latest status for each employee by simply selecting the row where rn = 1.

Joining with Employee Table:

After obtaining the last status for each employee, we perform an inner join with the Employee table on EmployeeId.

Filtering Active Status:

Finally, we filter to only return records where the status is Active.

Conclusion

In summary, querying for employees with specific conditions, like the latest active status, requires proper structuring of SQL queries. By leveraging ROW_NUMBER() along with JOIN, we can efficiently retrieve the relevant data.

Feel free to implement this approach in your SQL Server environment to enhance your skills and gain better insights from your database!

If you have any questions or further inquiries about queries, feel free to leave a comment below. Happy querying!