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N016-M3 Tier 2 · Core SQL · medium ecommerce · Brightlane

- `'high value domestic'` if the customer is US-based **and** has a city on record. - `'other'` for everyone else (non-US, or US with no city, or any other combination)

Part of CASE WHEN Expressions in SQL

The problem

Brightlane's marketing team is segmenting the customer base for a domestic campaign.

Write a query to return each customer's name, country, city, and a segment label:

  • 'high value domestic' if the customer is US-based and has a city on record.
  • 'other' for everyone else (non-US, or US with no city, or any other combination).

Assumptions:

  • The customers table contains every customer Brightlane has on file.
  • US customers are identified by country = 'US'.
  • Some customers have a recorded city value; others have city set to NULL.

Output:

  • One row per customer, with columns name, country, city, and segment.
Schema · ecommerce 5 tables
categories
id integer
name text
parent_id? integer
products
id integer
name text
category_id integer
price numeric
stock_qty integer
attributes? jsonb
order_items
id integer
order_id integer
product_id integer
quantity integer
unit_price numeric
customers
id integer
name text
email text
city? text
country text
created_at timestamptz
is_active boolean
orders
id integer
customer_id integer
ordered_at timestamptz
status text
total_amount numeric
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Solution query
SELECT
  name,
  country,
  city,
  CASE
    WHEN country = 'US'
    AND city IS NOT NULL THEN 'high value domestic'
    ELSE 'other'
  END AS segment
FROM
  customers

The shape

A single WHEN carries both segmentation rules joined by AND. A customer has to be US-based and have a city on record for the branch to match — anything short of both gets 'other'. The compound boolean lives inside the branch the same way it would inside a WHERE.

Clause by clause

  • SELECT name, country, city returns the three input columns the marketing team needs to see alongside the segment label. The raw fields make it possible to spot-check why a row landed in 'other'.
  • CASE WHEN country = 'US' AND city IS NOT NULL THEN 'high value domestic' ELSE 'other' END AS segment is the derived column. The WHEN is a compound boolean: both halves must be true for the branch to match. country = 'US' is the geography test; city IS NOT NULL is the data-completeness test. AND combines them into a single condition.
  • IS NOT NULL is the inverse of IS NULL — the only operator that returns a real boolean for the presence of a value. Writing it as city <> NULL would return NULL (PostgreSQL three-valued logic), and the branch would never match.
  • ELSE 'other' catches every customer who fails either half: non-US customers, US customers with no city on record, and any other combination.
  • FROM customers is the source set: every customer on file.

Why this and not two branches

The condition could be split: one branch for country = 'US', another for city IS NOT NULL, with the same THEN label on both. That doesn't work — "either condition matches" is OR, not AND, and US customers with no city would incorrectly land in 'high value domestic'. Compound conditions belong inside one branch, joined by the right boolean operator. The single-branch form makes the rule readable as a single sentence: US and has-a-city.

You practiced building a single WHEN condition from a compound boolean. AND, OR, and IS NULL all compose inside a CASE branch the same way they do in a WHERE clause — the branch is just a boolean expression that has to evaluate to true.

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