How to Segment Customers in SQL Using CASE WHEN Expressions

Data analyst Michael Nocito published a technical tutorial on August 8, 2026, explaining how to perform customer segmentation in SQL using CASE WHEN logic. The guide walks through aggregating a transactions table down to one row per customer, then applying a CASE expression to assign each customer a named group based on metrics like spend, purchase frequency, and recency. A sample dataset of 12 customers, 19 purchases, and $2,430 in total spend loaded into DuckDB is used throughout to demonstrate and verify each query. Nocito emphasizes that the grain must be correct before labelling — meaning data should first be aggregated to the customer level, then grouped by the resulting segment name. The tutorial also covers validation techniques, such as confirming all customers land in exactly one segment and that no rows fall into an unintended null group.