Compute a Difference (Delta) Between Two Columns on Different Rows

MySQL is an open-source, Relational Database Management System that stores data in a structured format using rows and columns. It’s software that enables users to create, manage, and manipulate databases. Similar to SQL we use queries to store and access data in the MySQL databases.

In this article, we’ll learn how to calculate the difference (delta) between two columns on different rows. Before moving into this article, you need to install MySQL. To do so you can refer to the below articles,

• Installation of MySQL Database server on Windows
• Installation of MySQL Database server on MacOs

Computing Difference (Delta) Between Two Columns on Different Rows

In MySQL, computing the difference between two columns on different rows can be done in multiple ways,

1. Using Window Functions
2. Using SELF JOIN
3. Using Subqueries
4. Using Common Table Expressions (CTEs)

First, we’ll create a table named geeks_table with two columns geek_id, and geek_value.

  CREATE TABLE geeks_table(
                geek_id SERIAL PRIMARY KEY, 
                geek_value INT
  );

Now we’ll insert values 100,200,300,400,500 into the table

  INSERT INTO geeks_table(geek_value) VALUES (100),(200),(300),(400),(500);

After execution of the above commands, geeks_table is ready with some data which is helpful for further computations. Now our table looks like,

geek_id	geek_value
1	100
2	200
3	300
4	400
5	500

Now here are some examples of computing the difference or delta between ‘geek_value‘ columns of different rows using ‘geeks_table‘ which we have created above.

1. Using Window Functions

Window functions in MySQL allow for performing calculations across a set of rows related to the current row. Using LAG(), and LEAD() window functions, we can compute the delta between values in two columns on different rows.

Use the below MySQL command to compute the delta between two columns on different rows using window functions.

1) Using LAG() window function:

 SELECT
      geek_value,
      geek_value - LAG(geek_value) OVER ( ORDER BY geek_id ) AS Delta
 FROM
      geeks_table;

Output:

geek_value	Deta
100	NULL
200	100
300	100
400	100
500	100

Explanation: LAG(geek_value) OVER (ORDER BY geek_id): This part of the query calculates the value of the ‘geek_value‘ column from the previous row, ordered by the ‘geek_id‘ column. The LAG() function retrieves the value from the previous row based on the specified ordering. If there’s no previous row, it returns NULL.

2) Using LEAD() window function:

 SELECT
       geek_value,
       geek_value - LEAD(geek_value) OVER ( ORDER BY geek_id ) AS Delta
 FROM
       geeks_table;

Output:

geek_value	Delta
100	-100
200	-100
300	-100
400	-100
500	NULL

Explanation: LEAD(geek_value) OVER (ORDER BY geek_id): This part of the query calculates the value of the ‘geek_value‘ column from the next or succeeding row, ordered by the geek_id column. The LEAD() function retrieves the value from the next or succeeding row based on the specified ordering. If there’s no succeeding row, it returns NULL.

2. Using SELF JOIN

Another approach for computing the delta between two columns on different rows is, by using self-join to retrieve the value of the previous row and then calculate the value of the difference or delta.

Use the below command to compute the delta between two columns on different rows using self-join.

SELECT 
       A.geek_value,
       A.geek_value - B.geek_value AS Delta
FROM 
       geeks_table A
JOIN
       geeks_table B ON A.geek_id = B.geek_id + 1;

Output:

geek_value	delta
200	100
300	100
400	100
500	100

Explanation:

• SELECT A.geek_value,A.geek_value – B.geek_value AS Delta: This part of the query selects the table’s first occurrence of geeks_table (A) geek_value column and then formulates the difference between the current row value (A.geek_value) and previous row value (B.geek_value).
• JOIN geeks_table B ON A.geek_id = B.geek_id + 1: This part of the query does the inner join or self join of A and B in both the table. It combines rows where the geek_id of A is the same as the geek_id of B incremented by 1.

3. Using Subqueries

Another approach for computing the delta between two columns on different rows is, by using subqueries to retrieve the value of the previous row and then calculate the value of the difference or delta.

Use the below command to compute the delta between two columns on different rows using subqueries.

SELECT
       A.geek_value,
       A.geek_value - (SELECT B.geek_value FROM geeks_table B WHERE B.geek_id + 1 = A.geek_id ) AS Delta
FROM
       geeks_table A;

Output:

geek_value	Delta
100	NULL
200	100
300	100
400	100
500	100

Explanation:

• (SELECT B.geek_value FROM geeks_table B WHERE B.geek_id + 1 = A.geek_id ): This kind of subqueries is called correlated subqueries. For every row value (A.geek_value) of geeks_table(A), this subquery returns the previous row value(B.geek_value) of geeks_table(B) by using the subsequent geek_id of the geeks_table.
• SELECT A.geek_value, A.geek_value – (SELECT B.geek_value FROM geeks_table B WHERE B.geek_id + 1 = A.geek_id ) AS Delta: This part of the query computes the difference between the first occurrence of geeks_table(A) geek_value column and value returned by subquery as aliases it as Delta.

4. Using Common Table Expressions (CTEs)

Other ways of computing delta between two columns on multiple rows is, by using common table expressions. CTEs are helpful in defining temporary result sets that can be referenced within a MySQL query.

Use the below command to compute the difference between two columns on multiple rows using CTE.

WITH previous_values AS (
          SELECT 
                      geek_id,
                      geek_value,
                      LAG(geek_value) OVER (ORDER BY geek_id) AS previous_value
          FROM
                     geeks_table
 )

SELECT 
            geek_id,
            geek_value,
            geek_value - previous_value AS Delta
FROM
            previous_values;

Output:

geek_id	geek_value	Delta
1	100	NULL
2	200	100
3	300	100
4	400	100
5	50	100

Explanation:

• WITH previous_values AS (….): This part of the query defines a Common Table Expression (CTE) named previous_values. CTEs allow you to create temporary result sets that can be referenced within the same query.
• SELECT geek_id, geek_value, LAG(geek_value) OVER (ORDER BY geek_id) AS previous_value FROM geeks_table: Inside the CTE, this query selects three columns (geek_id, geek_value, and value of previous_value) from the geeks_table. The LAG function retrieves the value of geek_value from the previous row based on the ordering specified by geek_id.
• SELECT geek_id, geek_value, geek_value – previous_value AS Delta FROM previous_values: This part of the query selects columns geek_id and geek_value from the previous_values CTE, and calculates the difference (delta) between the current geek_value and the previous row’s geek_value, and aliases the result as Delta.

Let us look into an example use case of yearly sales data, where we compute the difference in sales between each year using the above-said concept. The table considered here is ‘sales_table‘ having columns for date and sales. Now we compute the difference (delta) between sales of each year naming the column as Delta.

The table considered below is sales_table,

Date	sales
2019-02-11	3000
2020-03-14	2000
2021-01-24	6000
2022-04-05	3500
2023-06-18	5000

Now let us compute the difference between sales of each year as Delta using the window function LAG(),

 SELECT
      Date,
      sales - LAG(sales) OVER ( ORDER BY Date ) AS Delta
 FROM
      sales_table;

Output:

Date	Delta
2019-02-11	NULL
2020-03-14	-1000
2021-01-24	4000
2022-04-05	-2500
2023-06-18	1500

Conclusion

Therefore, In MySQL, we can compute the difference (delta) between two columns on different rows using window functions, self-join, subqueries, and CTEs common table expressions. Each approach may vary in computation speed and methodology. So one can choose ones own approach according to the requirement.