QuerydrillFirst payment per customer
An SQL exercise from Querydrill. Solve it with a SQL editor in your browser, checked on the rows it returns and again on changed data, and it is checked the moment you submit.
The task
From Sam: "For each customer who paid at least once, what was the date of their first paid payment? One row per email, cleaned to lowercase, oldest first, then by email." Return email and first_paid as YYYY-MM-DD.
What this exercise teaches
MIN on an ISO date string returns the earliest date, because YYYY-MM-DD text sorts in date order.
A hint if you get stuck
Group by the cleaned email and take MIN(substr(p.paid_at, 1, 10)). ISO dates sort correctly as text.
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