Get value from previous row data for next datas by dates - sql

Question

I have a table 2 transactions let's say table A and B, for some cases i need to transfer row datas from table B to A as new table with several conditions :
The price for the data transferred will follow from the previous data
The same date will not processed into results
When there is no previous data, it will not processed into results
For Example :
-----------Table A------------- ----------Table B----------
product | Date | Price | | Product | Date |
A | 2019-01-01 | 10 | | A | 2018-11-05 |
A | 2019-01-15 | 15 | | A | 2019-01-10 |
A | 2019-01-25 | 20 | | A | 2019-01-12 |
A | 2019-05-01 | 25 | | A | 2019-01-27 |
A | 2019-07-02 | 30 | | B | 2019-02-10 |
B | 2019-02-05 | 40 | | B | 2019-04-22 |
B | 2019-04-22 | 50 | | B | 2019-05-13 |
B | 2019-05-12 | 40 |
Result :
-----------Table C-------------
product | Date | Price |
A | 2019-01-01 | 10 |
A | 2019-01-10 | 10 | *The prices follow the data in the previous date (2019-01-01)
A | 2019-01-12 | 10 | *The prices follow the data in the previous date (2019-01-01)
A | 2019-01-15 | 15 |
A | 2019-01-25 | 20 |
A | 2019-01-27 | 20 | *The prices follow the data in the previous date (2019-01-25)
A | 2019-05-01 | 25 |
A | 2019-07-02 | 30 |
B | 2019-02-05 | 40 |
B | 2019-02-10 | 40 | *The prices follow the data in the previous date (2019-02-05)
B | 2019-04-22 | 50 |
B | 2019-05-12 | 40 |
B | 2019-05-13 | 40 | *The prices follow the data in the previous date (2019-05-12)
NOTE:
For product A in Table B on 2018-11-05 not processed into results because there's no data before that date in the table A for that product.
For product B in Table B on 2019-04-22 not processed into results because the date and product in table A and B are the same (The data is already in table A)
I try not to use looping mechanism because my data reaches millions, but i was too dizzy to think about it.

Answer 1

One way is using group by in a cte and then union:
WITH cte AS(
SELECT b.product,
b.[Date],
MAX(a.[Date]) AS [DateValue]
FROM TableA AS a
INNER JOIN TableB AS b ON a.product = b.product
WHERE a.[Date] <= b.[Date]
GROUP BY b.product, b.[Date]
)
SELECT *
FROM dbo.TableA AS a
UNION
SELECT b.product,
b.[Date],
a.Price
FROM cte AS c
INNER JOIN dbo.TableB AS b ON b.product = c.product AND b.[Date] = c.[Date]
INNER JOIN dbo.TableA AS a ON a.product = c.product AND a.[Date] = c.[DateValue]
ORDER BY product, [Date]

Answer 2

One method uses union all and cross apply:
select ab.product, ab.date, p.price
from ((select a.product, a.date
from a
) union -- intentional to remove duplicates
(select b.product b.date
from b
)
) ab cross apply
(select top (1) a.price
from a
where a.product = ab.product and a.date <= ab.date
order by ab.date desc
) p;
Note that cross apply will eliminate the rows from b that have no price.
If SQL support the ignore nulls option on either last_value() or lag(), this would be more appropriate with a full join:
select coalesce(a.product, b.product) as product,
coalesce(a.date, b.date) as date,
coalesce(a.price,
lag(ignore nulls a.price) over (partition by coalesce(a.product, b.product) order by coalesce(a.date, b.date)) as price
from a full join
b
on a.product = b.product and a.date = b.date;
Alas, SQL Server does not (currently) support that. You can make that work with a bit of effort and additional subqueries.

Answer 3

SQL MERGE is a very powerful tool to perform "CRUD" operation based on some condition...
Please follow the link for more details of this feature.
http://www.sqlservertutorial.net/sql-server-basics/sql-server-merge/
https://www.essentialsql.com/introduction-merge-statement/
Please feel free to ask if you have any doubt.