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N028-E1 Tier 3 · Intermediate · easy ecommerce · Brightlane

Return each customer's name and a display value for their city

Part of COALESCE and NULLIF in SQL

The problem

Brightlane's customer success team is preparing a regional outreach list and needs every customer to have a displayable city.

Write a query to return each customer's name and a display value for their city.

Assumptions:

  • The customers table has one row per customer with a name and a city.
  • Some customers have a missing city value because they registered without entering one.
  • A customer with a missing city should appear as 'Not on File'; all other customers should show their recorded city.

Output:

  • One row per customer, with columns name and city.
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,
  COALESCE(city, 'Not on File') AS city
FROM
  customers

The shape

COALESCE(city, 'Not on File') returns the recorded city for every customer who has one, and substitutes the string 'Not on File' only for the customers whose city is missing. The result is one populated value per row, which is what the outreach list needs.

Clause by clause

  • SELECT name, COALESCE(city, 'Not on File') AS city returns each customer's name alongside the substituted city value. The two arguments to COALESCE are checked left to right: if city is not NULL, it wins; if it is NULL, the literal 'Not on File' is returned instead. The alias AS city keeps the output column name matching the source.
  • FROM customers reads the customer records. There is no WHERE because every customer goes into the outreach list, including the ones with a missing city, which are exactly the rows the substitution is for.

Why this and not CASE WHEN city IS NULL THEN 'Not on File' ELSE city END

Both forms produce the same result on this data, and PostgreSQL rewrites COALESCE to that exact CASE expression internally. COALESCE(city, 'Not on File') is shorter, names the intent (first non-NULL wins), and reads as one decision rather than three clauses. Reach for CASE when the condition is more than a NULL check; for a straight NULL substitution, COALESCE is the right tool.

You practiced COALESCE(column, 'fallback') — substitute a display value when the column is missing so every output row carries something to show.

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