Who Actually Bought?

Kindled sends a hand-written thank-you card to everybody who has bought something, and the person addressing the envelopes needs names, not id numbers. The purchase is recorded in the activity table and the name lives on the account record, so neither table can produce the list on its own. Nobody may appear twice, however many times they have bought.

events — one row per tracked action. user_id points at a row in users. event_type is one of signup, view_product, add_to_cart and purchase. event_date is the day the action happened. Nothing guarantees a person produces all four kinds, or any particular number of rows.

users — one row per person who has created an account. source records how they arrived: organic means they found Kindled themselves, paid_search means they clicked a bought advert, and referral means an existing customer invited them. signup_date is the day they registered.

Both tables already exist in the database — there is nothing to create or load.

Task: Write a query that returns a single column, name, listing everybody who has recorded at least one purchase. Each person appears exactly once, in alphabetical sequence by name.

Example output — shape only, on invented people. If three of them had bought something, you would get: