Скачать или смотреть How to Subtract Values Between Two SQL Selections in MySQL

Learn how to effectively subtract values between two different selections in MySQL by leveraging SQL queries. We'll break down the process step-by-step for clarity.
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This video is based on the question https://stackoverflow.com/q/63945093/ asked by the user 'stressed out' ( https://stackoverflow.com/u/6547573/ ) and on the answer https://stackoverflow.com/a/63945203/ provided by the user 'Akina' ( https://stackoverflow.com/u/10138734/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Solving the SQL Challenge: Subtracting Values Between Two Selections

In the world of SQL, it's not uncommon to face scenarios where you need to analyze data from multiple selections and find differences between them. One such case could involve calculating the time difference between records from two similar SELECT queries, where both selections contain student_id, course_id, and created_at values, among others. In this guide, we will guide you through a structured approach to achieve this using MySQL.

The Problem at Hand

Imagine you have two separate SELECT queries that return data about students and their associated course statuses. For instance, consider the following outputs:

Query Output 1:

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Query Output 2:

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The Goal

Your objective is to write a SQL query that retrieves student_id, course_id, and calculates the time difference between the created_at values for records with matching student_id and course_id. This is a common challenge when joining temporary tables or datasets.

Crafting the SQL Query

To tackle this problem, we will use a JOIN operation to merge the two selections based on student_id and course_id. Following this, we will calculate the difference between the created_at columns from the two tables. Let’s break it down step-by-step.

Step 1: Define the Temporary Tables

Start by defining the datasets you are working with. In this case, we'll generate temporary representations of the two queries (let's call them t1 and t2):

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Step 2: Perform the JOIN

Now we can use a JOIN clause to combine these datasets based on the matching columns student_id and course_id:

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Step 3: Calculate the Time Difference

We can make use of the DATEDIFF function to calculate the difference between the created_at columns of the two selections:

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Here is the complete SQL query:

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Final Notes

By executing this query, you will successfully retrieve student_id, course_id, and the day difference between the created_at timestamps from both selections for matching students and courses. This approach not only provides clarity but also allows for effective data analysis, enhancing your SQL skills.

Conclusion

Understanding how to manipulate and query relational data is a crucial component of data analytics, especially when working with SQL databases. With the method discussed above, you should feel more confident tackling similar problems in the future. Don't hesitate to experiment and modify the query to fit your specific needs!