Adding a New Column in SQL Corresponding to Previous Column

Introduction

In this article, we will explore how to add a new column in an existing SQL table that corresponds to the previous row’s value. We’ll use MySQL 8+ as our database management system and provide a solution using Common Table Expressions (CTEs) to achieve this.

Understanding the Problem

The problem at hand is to create a new column timetable that indicates whether two consecutive rows belong to the same “timetable” or not. In other words, we want to check if the user who started the project in the previous row has also started a new project within 20 minutes.

To solve this problem, we need to identify the relationship between consecutive rows and determine when the user is considered part of the same “timetable”.

Using Common Table Expressions (CTEs)

A CTE is a temporary result set that can be referenced within a SELECT, INSERT, UPDATE, or DELETE statement. In our case, we’ll use a CTE to simplify the query and make it more readable.

How it works

1. We create a CTE cte with a subquery that selects all columns from the original table (test) along with a new column new_timetable.
2. The new_timetable column is calculated using a CASE statement, which checks if the current row’s name and date_time are the same as the previous row’s name and date_time, respectively.
3. If they are the same, we set new_timetable to 0, indicating that the user belongs to the same “timetable”. Otherwise, we set it to 1.
4. We select all columns from the CTE, including the new timetable column, which is calculated using a window function (SUM).

MySQL Query

WITH cte AS (
SELECT *, 
CASE WHEN name = LAG(name) OVER (ORDER BY name, date_time)
    AND TIMESTAMPDIFF(MINUTE, LAG(date_time) OVER (ORDER BY name, date_time), date_time) < 20
   THEN 0
   ELSE 1
   END new_timetable
FROM test
)
SELECT name, date_time, SUM(new_timetable) OVER (ORDER BY name, date_time) timetable
FROM cte
ORDER BY name, date_time;

How it Works

Let’s break down the query step by step:

• The LAG function is used to access the previous row’s values.
• The TIMESTAMPDIFF function calculates the difference between two timestamps in minutes.
• In the CASE statement, we check if the current row’s name and date_time are the same as the previous row’s name and date_time, respectively. We also check if the timestamp difference is less than 20 minutes using TIMESTAMPDIFF.
• If the conditions are met, we set new_timetable to 0, indicating that the user belongs to the same “timetable”. Otherwise, we set it to 1.
• The window function (SUM) aggregates the values of new_timetable for each row, giving us the final result.

Example Use Case

Suppose we have the following table:

Running the query will produce the following result:

Conclusion

In this article, we demonstrated how to add a new column in SQL that corresponds to the previous row’s value. We used Common Table Expressions (CTEs) to simplify the query and make it more readable.

By using CTEs, we can break down complex queries into smaller, more manageable pieces, making them easier to understand and maintain. The example use case shows how this technique can be applied in real-world scenarios to extract valuable insights from data.

We hope this article has provided you with a solid understanding of how to create new columns based on previous values using SQL and CTEs. Happy querying!

Last modified on 2023-08-22