Скачать или смотреть Joining a Table with the Next Row in SQL Server

Learn how to perform a self-join in SQL Server to compare rows with the `next employee` in your dataset. This guide breaks down the implementation step-by-step.
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Joining a Table with the Next Row in SQL Server: A Complete Guide

When working with databases, you may often need to compare data across different rows within the same table. A common requirement is to join a table on itself to access information from the next row. In this guide, we will explore how to perform a self-join in SQL Server using a practical example involving employee work hours.

The Problem

Imagine you have a table storing employee data that includes their Empno, Work_hours, and Max_Hours. You want to compare each employee's details with the next employee in the list, sorted by their employee number (Empno). Here's a small example dataset:

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

The goal is to create a new dataset that displays each employee alongside the subsequent employee’s work hours and maximum hours. The expected output should look like this:

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

The Solution

To achieve this output, we will utilize the LEAD() window function in SQL Server. This function allows you to access data from the next row based on a specified order, which is perfect for our use case. Below, we will break down the solution into clear sections: creating the table, inserting data, and writing the query.

Step 1: Create a Temporary Table

First, we need to create a temporary table that will hold our employee data. We can use the following SQL commands:

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

Step 2: Insert Sample Data

Next, we will insert sample employee records into the temporary table:

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

Step 3: Write the Self-Join Query

Now we can write the query that will allow us to join the table with its next row. We will use the LEAD() function to fetch the employee number, work hours, and maximum hours of the next employee:

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

Step 4: Output Explanation

When you run the above query, you’ll receive the following output:

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

This output matches our expectations—it shows each employee's data next to the next employee's details, with the last row containing NULL values as there is no subsequent employee.

Conclusion

Using the LEAD() function in SQL Server is a powerful way to perform a self-join with the next row in your dataset. By following the steps outlined above, you can easily compare and contrast data across rows within the same table. Remember, this technique doesn't only apply to employee data—it can be utilized in various scenarios where row-by-row comparisons are necessary.

Feel free to explore this method in your own projects, and don't hesitate to reach out if you have any questions or need further clarification!