Add additional column to query based on dates in another table - sql-server-2008

Question

I'm looking to query the datatbl table and return the table with an additional column based on the Start_date values of the productstbl table.
Below I have created a schema with the initial table (datatbl_before) and the desired end result (datatbl_after), where a Product_Lookup column has been added to the datatbl table and shows the corresponding Product from the productstbl based on the Asset_ID of the line and the Start_date.
For example, in the first row of datatbl, the row relates to Asset_ID '1' and occurred on 2017-08-16, this would retrieve the product value 'A' from the productstbl table as it is after the start_date value 2017-08-12, but before the next start_date value of 2017-09-27 related to that Asset_ID .
In the case where there is no proceeding date for the Asset_ID lookup and start_date, a 'NONE' value could be returned - e.g. Row 5 of datatbl
Can anyone provide any direction or advice on how to tackle this one?
Many thanks in advance!
CREATE TABLE datatbl_before (
PDate DATE,
Asset_ID varchar(255),
Rev int
);
CREATE TABLE datatbl_after (
PDate DATE,
Asset_ID varchar(255),
Rev int,
Product_Lookup varchar(255)
);
CREATE TABLE productstbl (
Asset_ID varchar(255),
Start_date DATE,
Product varchar(255)
);
INSERT INTO productstbl
VALUES
(' 1 ',' 2017-08-12' ,' A' ),
(' 1' ,' 2017-09-27' ,' B' ),
(' 1' ,' 2018-02-14' ,' C' ),
(' 2' ,' 2018-01-17' ,' A' ),
(' 2' ,' 2018-02-21' ,' B' ),
(' 2' ,' 2018-03-05' ,' C' );
INSERT INTO datatbl_before
VALUES
(' 2017-08-16' ,' 1' ,' 10'),
(' 2017-09-29' ,' 1' ,' 15'),
(' 2018-02-13' ,' 1' ,' 12'),
(' 2018-02-19' ,' 1' ,' 10'),
(' 2018-01-01' ,' 2' ,' 12'),
(' 2018-01-25' ,' 2' ,' 33'),
(' 2018-02-25' ,' 2' ,' 67'),
(' 2018-03-07' ,' 2' ,' 71');
INSERT INTO datatbl_after
VALUES
(' 2017-08-16' ,' 1' ,' 10' ,' A' ),
(' 2017-09-29' ,' 1' ,' 15' ,' B' ),
(' 2018-02-13' ,' 1' ,' 12' ,' B' ),
(' 2018-02-19' ,' 1' ,' 10' ,' C' ),
(' 2018-01-01' ,' 2' ,' 12' ,' NONE'),
(' 2018-01-25' ,' 2' ,' 33' ,' A' ),
(' 2018-02-25' ,' 2' ,' 67' ,' B' ),
(' 2018-03-07' ,' 2' ,' 71' ,' C' );

Answer 1

You can use a sub query, which goes and selects the data you want, using data from the outer query in the where clause. In this case, it finds all the products that are the right asset, and started before or on the transaction date, sorts them so the most recent one is first (the one we want) and takes only that one (The TOP 1 part).
The ISNULL part changes any missing values from NULL to 'NONE'
Select DB.*,
ISNULL(
(Select top 1 product
from productstbl
where Asset_ID = DB.Asset_ID
and Start_date <= DB.PDate
order by Start_date desc
)
, 'NONE') as Product_Lookup
from datatbl_before DB;