Selecting unique records based on date of effect, ending on date of discontinue - sql-server

Question

I have an interesting conundrum and I am using SQL Server 2012 or SQL Server 2016 (T-SQL obviously). I have a list of products, each with their own UPC code. These products have a discontinue date and the UPC code gets recycled to a new product after the discontinue date. So let's say I have the following in the Item_UPCs table:
Item Key | Item Desc | UPC | UPC Discontinue Date
123456 | Shovel | 0009595959 | 2018-04-01
123456 | Shovel | 0007878787 | NULL
234567 | Rake | 0009595959 | NULL
As you can see, I have a UPC that gets recycled to a new product. Unfortunately, I don't have an effective date for the item UPC table, but I do in an items table for when an item was added to the system. But let's ignore that.
Here's what I want to do:
For every inventory record up to the discontinue date, show the unique UPC associated with that date. An inventory record consists of the "Inventory Date", the "Purchase Cost", the "Purchase Quantity", the "Item Description", and the "Item UPC".
Once the discontinue date is over with (e.g.: it's the next day), start showing only the UPC that is in effect.
Make sure that no duplicate data exists and the UPCs are truly being "attached" to each row per whatever the date is in the query.
Here is an example of the inventory details table:
Inv_Key | Trans_Date | Item_Key | Purch_Qty | Purch_Cost
123 | 2018-05-12 | 123456 | 12.00 | 24.00
108 | 2018-03-22 | 123456 | 8.00 | 16.00
167 | 2018-07-03 | 234567 | 12.00 | 12.00
An example query:
SELECT DISTINCT
s.SiteID
,id.Item_Key
,iu.Item_Desc
,iu.Item_Department
,iu.Item_Category
,iu.Item_Subcategory
,iu.UPC
,iu.UPC_Discontinue_Date
,id.Trans_Date
,id.Purch_Cost
,id.Purch_Qty
FROM Inventory_Details id
INNER JOIN Item_UPCs iu ON iu.Item_Key = id.Item_Key
INNER JOIN Sites s ON s.Site_Key = id.Site_Key
The real query I have is far too long to post here. It has three CTEs and the resultant query. This is simply a mockup. Here is an example result set:
Site_ID | Item_Key | Item_Desc | Item_Department | Item_Category | UPC | UPC_Discontinue Date | Trans_Date | Purch_Cost | Purch_Qty
2457 | 123456 | Shovel | Digging Tools | Shovels | 0009595959 | 2018-04-01 | 2018-03-22 | 16.00 | 8.00
2457 | 123456 | Shovel | Digging Tools | Shovels | 0007878787 | NULL | 2018-03-22 | 16.00 | 8.00
2457 | 234567 | Rakes | Garden Tools | Rakes | 0009595959 | NULL | 2018-07-03 | 12.00 | 12.00
2457 | 123456 | Shovel | Digging Tools | Shovels | 0007878787 | NULL | 2018-05-12 | 24.00 | 12.00
Do any of you know how I can "assign" a UPC to a specific range of dates in my query and then "assign" an updated UPC to the item for every effective date thereafter?
Many thanks!

Answer 1

Given your current Item_UPC table, you can generate effective start dates from the Discontinue Date using the LAG analytic function:
With Effective_UPCs as (
select [Item_Key]
, [Item_Desc]
, [UPC]
, coalesce(lag([UPC_Discontinue_Date])
over (partition by [Item_Key]
order by coalesce( [UPC_Discontinue_Date]
, datefromparts(9999,12,31))
),
lag([UPC_Discontinue_Date])
over (partition by [UPC]
order by coalesce( [UPC_Discontinue_Date]
, datefromparts(9999,12,31))
)) [UPC_Start_Date]
, [UPC_Discontinue_Date]
from Item_UPCs i
)
select * from Effective_UPCs;
Which yields the following Results:
| Item_Key | Item_Desc | UPC | UPC_Start_Date | UPC_Discontinue_Date |
|----------|-----------|------------|----------------|----------------------|
| 123456 | Shovel | 0007878787 | 2018-04-01 | (null) |
| 123456 | Shovel | 0009595959 | (null) | 2018-04-01 |
| 234567 | Rake | 0009595959 | 2018-04-01 | (null) |
This function produces a fully open ended interval where both the start and discontinue dates could be null indicating that it's effective for all time. To use this in your query simply reference the Effective_UPCs CTE in place of the Item_UPCs table and add a couple additional predicates to take the effective dates into consideration:
SELECT DISTINCT
s.SiteID
,id.Item_Key
,iu.Item_Desc
,iu.Item_Department
,iu.Item_Category
,iu.Item_Subcategory
,iu.UPC
,iu.UPC_Discontinue_Date
,id.Trans_Date
,id.Purch_Cost
,id.Purch_Qty
FROM Inventory_Details id
INNER JOIN Effective_UPCs iu
ON iu.Item_Key = id.Item_Key
and (iu.UPC_Start_Date is null or iu.UPC_Start_Date < id.Trans_Date)
and (iu.UPC_Discontinue_Date is null or id.Trans_Date <= iu.UPC_Discontinue_Date)
INNER JOIN Sites s ON s.Site_Key = id.Site_Key
Note that the above query uses a partially open range (UPC_Start_Date < trans_date <= UPC_Discontinue_Date instead of <= for both inequalities) this prevents transactions occurring exactly on the discontinue date from matching both the prior and next Item_Key record. If transactions that occur exactly on the discontinue date should match the new record and not the old simply swap the two inequalities:
and (iu.UPC_Start_Date is null or iu.UPC_Start_Date <= id.Trans_Date)
and (iu.UPC_Discontinue_Date is null or id.Trans_Date < iu.UPC_Discontinue_Date)
instead of
and (iu.UPC_Start_Date is null or iu.UPC_Start_Date < id.Trans_Date)
and (iu.UPC_Discontinue_Date is null or id.Trans_Date <= iu.UPC_Discontinue_Date)