Learn CASE WHEN in Aggregations | CASE WHEN for Business Logic

When you need to calculate custom business metrics, combining CASE WHEN with SQL aggregation functions is a powerful approach. This technique lets you create flexible key performance indicators (KPIs) directly in your queries, such as counting only completed orders or summing only high-value transactions. By embedding conditional logic within aggregate functions, you can tailor your analysis to answer specific business questions.


              12
            
SELECT COUNT(CASE WHEN status_id = 4 THEN 1 END) AS completed_orders
FROM orders;

In this example, you are counting only the orders that have a status_id of 4, which corresponds to the "Delivered" status in the order_statuses table. The CASE WHEN clause checks each row: if the condition is true, it returns 1; otherwise, it returns NULL. The COUNT() function then counts only the non-NULL values, giving you the total number of completed orders.


              12345
            
SELECT
  region_id,
  SUM(CASE WHEN total_amount > 100 THEN total_amount ELSE 0 END) AS high_value_sales
FROM orders
GROUP BY region_id;

Here, you are calculating the total sales amount for each region, but only including orders where the total_amount is greater than 100. The CASE WHEN expression returns the order's total_amount only if it meets the threshold; otherwise, it returns 0. The SUM() function aggregates these values by region, so you see high-value sales broken down by area.

Definition

Conditional aggregation uses CASE WHEN statements inside aggregate functions such as SUM(), COUNT(), or AVG() to perform calculations only for rows that meet specific criteria. This is essential in business intelligence for building metrics like conversion rates, segment-specific totals, or custom KPIs directly in your SQL queries.

To use CASE WHEN with aggregation functions effectively, follow these best practices:

Always return a value or NULL from the CASE WHEN expression, depending on what you want to count or sum;
Use COUNT(CASE WHEN condition THEN 1 END) to count only rows matching a condition;
Use SUM(CASE WHEN condition THEN value ELSE 0 END) to total values only for specific cases;
Use AVG(CASE WHEN condition THEN value END) to average values conditionally, ignoring NULL results;
Make sure your conditions align with business logic and are easy to read for future maintenance.

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Section 2. Chapter 5

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              12
            
SELECT COUNT(CASE WHEN status_id = 4 THEN 1 END) AS completed_orders
FROM orders;


              12345
            
SELECT
  region_id,
  SUM(CASE WHEN total_amount > 100 THEN total_amount ELSE 0 END) AS high_value_sales
FROM orders
GROUP BY region_id;

Definition

To use CASE WHEN with aggregation functions effectively, follow these best practices:

Always return a value or NULL from the CASE WHEN expression, depending on what you want to count or sum;
Use COUNT(CASE WHEN condition THEN 1 END) to count only rows matching a condition;
Use SUM(CASE WHEN condition THEN value ELSE 0 END) to total values only for specific cases;
Use AVG(CASE WHEN condition THEN value END) to average values conditionally, ignoring NULL results;
Make sure your conditions align with business logic and are easy to read for future maintenance.

Everything was clear?

Thanks for your feedback!

Section 2. Chapter 5

CASE WHEN in Aggregations

1. How does CASE WHEN enhance aggregation functions?

2. What is conditional aggregation?

3. How would you count only completed orders using CASE WHEN?

CASE WHEN in Aggregations

1. How does CASE WHEN enhance aggregation functions?

2. What is conditional aggregation?

3. How would you count only completed orders using CASE WHEN?