SELECT
  v.sku,
  sum(s.quantity) AS quantity_sold
FROM
    spree_variants v INNER JOIN
    spree_line_items s ON s.variant_id = v.id INNER JOIN
    spree_orders o ON s.order_id = o.id INNER JOIN
WHERE
    o.completed_at > (CURRENT_DATE - 90) AND
    o.state = 'complete'
GROUP BY
    v.sku
ORDER BY
    sum(s.quantity) DESC