SELECT a.date AS date,
       a.id AS id,
       p.price AS price
  FROM articles a
  LEFT JOIN (SELECT date,
                    id,
                    price
               FROM prices) p
    ON a.date = p.date
   AND a.id = p.id

WITH T AS 
(
SELECT c1 as date, c2 as id, c3 as price
FROM VALUES 
(
  (toDate ('2019-08-17'), 1, 5) 	
, (toDate ('2019-08-17'), 2, 15.4) 	
, (toDate ('2019-08-18'), 1, 0) 	
, (toDate ('2019-08-18'), 2, 0) 	
, (toDate ('2019-08-19'), 1, 0) 	
, (toDate ('2019-08-19'), 2, 0) 	
, (toDate ('2019-08-20'), 1, 0) 	
, (toDate ('2019-08-20'), 2, 0) 	
, (toDate ('2019-08-21'), 1, 5.6) 	
, (toDate ('2019-08-21'), 2, 14) 	
)
)
SELECT 
  *
, last_value (nullif (price, 0)) over (partition by id order by date)
	as prev_value
, last_value (nullif (price, 0)) over (partition by id order by date desc)
	as next_value
FROM T
ORDER BY date, id