SQL : automatically fill price between dates - sql

Question

I'm trying to write a view from two tables, one referential table that contains products ID and weeks :
+------------+------+
| Product_id | week | t1
+------------+------+
| 1 | 1 |
| 2 | 1 |
| 1 | 2 |
| 2 | 2 |
| 1 | 3 |
| 2 | 3 |
+------------+------+ etc...
the other one contains Products ID, weeks when the product's price changed and the price
+------------+------+-------+
| Product_id | week | price | t2
+------------+------+-------+
| 1 | 1 | 70 |
| 1 | 2 | 50 |
| 2 | 2 | 70 |
| 1 | 4 | 30 |
| 2 | 4 | 40 |
+------------+------+-------+
I know how to achieve easily this by joining the two tables :
+------------+------+-------+
| Product_id | week | price |
+------------+------+-------+
| 1 | 1 | 70 |
| 1 | 2 | 50 |
| 1 | 3 | |
| 1 | 4 | 30 |
| 1 | 5 | |
| 2 | 1 | |
| 2 | 2 | 70 |
| 2 | 3 | |
| 2 | 4 | 40 |
| 2 | 5 | |
+------------+------+-------+
But my goals would rather be to fill in the gaps and have the price for each week (without creating any new table), as such :
+------------+------+-------+
| Product_id | week | price |
+------------+------+-------+
| 1 | 1 | 70 |
| 1 | 2 | 50 |
| 1 | 3 | 50 |
| 1 | 4 | 30 |
| 1 | 5 | 30 |
| 2 | 1 | |
| 2 | 2 | 70 |
| 2 | 3 | 70 |
| 2 | 4 | 40 |
| 2 | 5 | 40 |
+------------+------+-------+ (product 2 isn't sold yet at week 1, so it doesn't have a price).
I can't see how I would do this in SQL. I haven't used PARTITION BY or LAG yet, and it might be what I'm looking for. If anyone can push me in the right direction, I would appreciate it :)

Answer 1

You can use window functions - the ignore nulls clause, which teradata supports, comes handy here:
select
t1.product_id,
t1.week,
coalesce(
t2.price,
lag(t2.price ignore nulls) over(partition by t1.product_id order by t1.week)
) price
from t1
left join t2
on t2.product_id = t1.product_id
and t2.week = t1.week
Or better yet, as suggested by dnoeth, you can use last_value(), which avoids the need for coalesce():
select
t1.product_id,
t1.week,
last_value(t2.price ignore nulls) over(partition by t1.product_id order by t1.week) price
from t1
left join t2
on t2.product_id = t1.product_id
and t2.week = t1.week

Answer 2

Use a cross join to generate the rows, then left join and window functions:
with weeks as (
select row_number() over (order by product_id) as n
from table1
)
select t1.product_id, w.n as week,
coalesce(t2.price, lag(t2.price ignore nulls) over (partition by p.product_id order by w.n)
) as price
from (select distinct product_id
from table1 t1
) p cross join
weeks w left join
table2 t2
on t2.product_id = p.product_id and t2.week = w.week
where w.n <= 5

Answer 3

You can do this with a LEFT JOIN.
SELECT t1.Product_id, t1.week, tmp.price
FROM t1
LEFT JOIN t2 tmp ON tmp.Product_id = t1.Product_id AND
tmp.week = (SELECT MAX(week) FROM t2
WHERE Product_id = tmp.Product_id AND week <= t1.week)
ORDER BY t1.Product_id, t1.week
I would argue it's cleaner yet with OUTER APPLY, but I don't know if that's supported by teradata.
SELECT t1.Product_id, t1.week, oa.price
FROM t1
OUTER APPLY (SELECT TOP 1 price FROM t2
WHERE Product_id = t1.Product_id AND week <= t1.week
ORDER BY week DESC) oa
ORDER BY t1.Product_id, t1.week