Update a column with LastExclusionDate - sql-server

Question

In SQL Server 2012, I have a table t1 where we store a list of excluded product.
I would like to add a column LastExclusionDate to store the date since when the product has been excluded.
Every day the product is inserted into the table if it is excluded. If not there will be no row and the next time when the product will be excluded there will be a gap date with the previous insert.
I would like to find a T-SQL query to update the LastExclusionDate column.
I would like to use it to populate column LastExclusionDate the first time (=initialisation) and use it every day to update the column when we insert a new row
I've tried this query, but I don't know how to get LastExclusionDate!
;WITH Cte AS
(
SELECT
product_id,
CreationDate,
LAG(CreationDate) OVER (PARTITION BY Product_ID ORDER BY CreationDate) AS GapStart,
(DATEDIFF(DAY, LAG(CreationDate) OVER (PARTITION BY Product_id ORDER BY CreationDate), CreationDate) -1) AS GapDays
FROM
#t1
)
SELECT *
FROM cte
Here's some sample data:
+------------+--------------+--------------------------------+
| product_id | CreationDate | LastExclusionDate_(toPopulate) |
+------------+--------------+--------------------------------+
| 100 | 2018-05-01 | 2018-05-01 |
| 100 | 2018-05-02 | 2018-05-01 |
| 100 | 2018-05-03 | 2018-05-01 |
| 100 | 2018-06-01 | 2018-06-01 |
| 100 | 2018-06-02 | 2018-06-01 |
| 200 | 2018-09-01 | 2018-09-01 |
| 200 | 2018-09-02 | 2018-09-01 |
| 200 | 2018-09-17 | 2018-09-17 |
+------------+--------------+--------------------------------+
Thanks

Answer 1

The idea in finding gap-less sequences is to compare the series to a gap-less sequence and find groups of records where the difference of both doesn't change. For example, when the date increases one by one and a row number also does, then the difference between both stays the same and we found a group:
WITH
cte (product_id, CreationDate, grp) AS (
SELECT product_id, CreationDate
, DATEDIFF(day, '19000101', CreationDate)
- ROW_NUMBER() OVER (PARTITION BY product_id ORDER BY CreationDate)
FROM #t1
)
SELECT product_id, CreationDate
, MIN(CreationDate) OVER (PARTITION BY product_id, grp) AS LastExclusionDate
FROM cte

Answer 2

For ongoing daily insertions it can be done with something like this.
INSERT INTO <yourTable>
SELECT
newProduct.[product_id],
newProduct.[creationDate],
isnull(existingProduct.[lastExclusionDate], newProduct.[creationDate]) AS [lastExclusionDate]
FROM
(SELECT <#product_id> AS [product_id], <#createionDate> AS [creationDate]) AS newProduct
LEFT JOIN #temp existingProduct
ON existingProduct.[product_id] = newProduct.product_id
AND existingProduct.[creationDate] = DATEADD(DAY,-1,newProduct.[creationDate])
I've got a demo here http://rextester.com/BDEO23118 . It's a larger than necessary demo because it uses the code above with the data you provided to populate a table row-by-row like you might in a daily update process. It then does individual insertions using this code with some new dates so you can see the way it handles new ranges. (just an FYI, rextester displays result dates in day.month.year hh:mm:ss format, but you can dump the script into management studio and it will output in DATE format)