Customers in the US — WHERE Clause in SQL

The problem

Brightlane's marketing team is preparing a domestic email campaign for the upcoming product launch and needs a list of US-based customers to target.

Write a query to return the name and email of every customer registered in the United States.

Assumptions:

The customers table contains every customer Brightlane has on file.
The country column records the customer's country of registration as a two-letter code; US-registered customers have country set to 'US'.

Output:

One row per US-registered customer, with columns name and email.

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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Worked solution Try it yourself first

Solution query

SELECT
  name,
  email
FROM
  customers
WHERE
  country = 'US'

The shape

The equality check on country decides which rows survive, and the SELECT list controls what comes back for each one. Filter first, shape second.

Clause by clause

SELECT name, email names two columns from the customers table — exactly what the marketing email campaign needs, nothing more. The columns come back in the order they're listed.
FROM customers is the source: every customer Brightlane has on file, with country recorded as a two-letter code.
WHERE country = 'US' keeps only the rows whose country value is the literal 'US'. PostgreSQL checks each row's country against the literal once, in isolation, and either keeps the row or drops it before SELECT ever sees it.

The trap

String comparisons in PostgreSQL are case-sensitive and whitespace-sensitive. WHERE country = 'US' matches 'US' exactly; it does not match 'us', 'Us', or 'US ' with a trailing space. Whenever the values in a column may have been entered inconsistently, an exact-match filter will silently miss the ones that don't match the spelling you wrote.

You practiced filtering rows by an exact-match condition on a single column. The WHERE column = 'value' shape is how every constant-filter query starts.

Return the name and email of every customer registered in the United States

The shape

Clause by clause

The trap

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