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SELECT DISTINCT user_id, 
CAST(DATE_TRUNC('month',paid_at) as date) as month,
MIN(CAST(DATE_TRUNC('month',paid_at) as DATE)) OVER (partition by user_id) as first_paid_at 
FROM tools_shop.orders
WHERE CAST(DATE_TRUNC('YEAR',paid_at) as date) = '2020-01-01'