Conversion funnel from view to cart to purchase

Table events:

A user moves through a funnel: view, then cart, then purchase, each step strictly later than the previous step (a step counts only after the user's first view, then the first cart after that, then a purchase after that cart). Events can repeat.

Return one row with the number of distinct users who reached each step.

Output columns: viewed, carted, purchased.

CREATE TABLE events (
  user_id int NOT NULL,
  event_name text NOT NULL,
  created_at timestamp NOT NULL
);