# Calculating Year-to-Year and Month-to-Month Differences Using SQL

## Understanding Year-to-Year and Month-to-Month Differences using SQL

If you are a data analyst, you understand the importance of business metrics and periodic growth. Businesses rely on key performance indicators (KPIs) to evaluate their performance over time.

KPIs are objective measurements that enable stakeholders to evaluate the success of a business strategy by comparing it against goals set for a specific period. However, data analysts need specific tools to calculate year-to-year and month-to-month differences that show whether a business is achieving its goals.

This is where SQL comes in. SQL is a powerful programming language that allows data analysts to calculate business metrics accurately.

In this article, we will explore SQL’s capabilities and how it can help us understand business metrics better by calculating year-over-year (YOY) metrics, month-over-month (MOM) metrics, and quarter-over-quarter (QOQ) metrics.

## Understanding Year-Over-Year Metrics from a Business Perspective

YOY metrics are essential in evaluating a business’s performance over time. By comparing the same period in different years, analysts can form an idea of a business’s overall long-term growth rate.

For example, if a business had a revenue of \$1 million in 2019 and \$1.2 million in 2020, its YOY growth rate is 20%. YOY metrics help analysts understand trends, and they are valuable in identifying potential issues and opportunities.

However, calculating YOY metrics using SQL requires specific tools and knowledge, including window functions and the LAG function.

## How to Calculate YOY Metrics with SQL

SQL’s window functions allow data analysts to calculate YOY metrics without the need to write complex queries. Window functions enable analytical calculations to be performed on a set of records.

### Here is an example of a query that calculates YOY difference using window function:

SELECT date,
sales,
LAG(sales, 1, NULL) OVER (ORDER BY date) AS prev_sales,
(sales - LAG(sales, 1, NULL) OVER (ORDER BY date)) /
LAG(sales, 1, NULL) OVER (ORDER BY date) AS yoy_diff
FROM daily_metrics;

The above query selects data from a table named daily_metrics and calculates YOY difference by comparing the sales figures from the current year with the previous year’s corresponding sales figure. Note that the LAG function allows us to compare the current year’s sales with the previous year’s sales.

It also enables us to control how many rows back we want to go. In this example, we are comparing the current year’s sales with the previous year’s corresponding sales figure.

## Calculating Month-to-Month and Quarter-to-Quarter Differences

MOM and QOQ metrics are also vital in evaluating business performance over time. MOM metrics analyze a business’s performance from one month to another, while QOQ metrics evaluate performance from one quarter to another.

Calculating these metrics requires using a Common Table Expression (CTE) and comparing the current quarter or month with the previous quarter or month. Here is an example of a query that calculates MOM and QOQ difference using a CTE in SQL:

WITH monthly_metrics AS (
SELECT
date,
sales,
LAG(sales, 1, NULL) OVER (ORDER BY date) AS prev_sales,
EXTRACT(month FROM date) AS month
FROM daily_metrics
)
SELECT
month,
AVG(sales) AS avg_sales,
SUM(sales) AS total_sales,
AVG(sales) -
LAG(AVG(sales), 1, NULL) OVER (ORDER BY month) AS mom_diff,
AVG(sales) -
LAG(AVG(sales), 3, NULL) OVER (ORDER BY month) AS qoq_diff

FROM monthly_metrics
GROUP BY month;

In the above query, we have created a CTE named monthly_metrics that selects data from a table named daily_metrics and extracts the month from the date. We then calculate the monthly average, total sales, and MOM and QOQ differences using window functions and the LAG function.

What’s Next? SQL offers powerful tools for data analysts to solve data problems, and window functions are an essential aspect of these tools.

However, SQL’s capabilities go beyond the aforementioned topics. If you want to learn more about SQL’s window functions, knowing how it functions arithmetically and it’s over clause would be a great step towards mastering advanced SQL concepts.

You can use a SQL resource, such as a cheat sheet, to supplement your knowledge. With this, data analysts can better meet the changing needs of their businesses.

In conclusion, SQL offers powerful tools for data analysts to calculate year-to-year and month-to-month differences that help stakeholders evaluate a business’s performance over time. Calculating business metrics accurately requires specific knowledge of SQL’s window functions, such as LAG and LEAD, which allow analysts to compare the same period in different years.

SQL’s Common Table Expression (CTE) offers an effective way to calculate MOM and QOQ metrics by comparing the current quarter or month with the previous quarter or month. By mastering SQL’s advanced concepts, data analysts can better meet the changing needs of their businesses and provide valuable insights that lead to better decision-making.

Incorporating a SQL resource, such as a cheat sheet, can help data analysts stay up-to-date on their knowledge of SQL’s tools and functions, making them more effective in their roles.