Checking Slowly Changing Dimension 2 - db2

Question

I have a table that looks like this:
A slowly changing dimension type 2, according to Kimball.
Key is just a surrogate key, a key to make rows unique.
As you can see there are three rows for product A.
Timelines for this product are ok. During time the description of the product changes.
From 1-1-2020 up until 4-1-2020 the description of this product was ProdA1.
From 5-1-2020 up until 12-2-2020 the description of this product was ProdA2 etc.
If you look at product B, you see there are gaps in the timeline.
We use DB2 V12 z/Os. How can I check if there are gaps in the timelines for each and every product?
Tried this, but doesn't work
with selectie (key, tel) as
(select product, count(*)
from PROD_TAB
group by product
having count(*) > 1)
Select * from
PROD_TAB A
inner join selectie B
on A.product = B.product
Where not exists
(SELECT 1 from PROD_TAB C
WHERE A.product = C.product
AND A.END_DATE + 1 DAY = C.START_DATE
)
Does anyone know the answer?

Answer 1

The following query returns all gaps for all products.
The idea is to enumerate (RN column) all periods inside each product by START_DATE and join each record with its next period record.
WITH
/*
MYTAB (PRODUCT, DESCRIPTION, START_DATE, END_DATE) AS
(
SELECT 'A', 'ProdA1', DATE('2020-01-01'), DATE('2020-01-04') FROM SYSIBM.SYSDUMMY1
UNION ALL SELECT 'A', 'ProdA2', DATE('2020-01-05'), DATE('2020-02-12') FROM SYSIBM.SYSDUMMY1
UNION ALL SELECT 'A', 'ProdA3', DATE('2020-02-13'), DATE('2020-12-31') FROM SYSIBM.SYSDUMMY1
UNION ALL SELECT 'B', 'ProdB1', DATE('2020-01-05'), DATE('2020-01-09') FROM SYSIBM.SYSDUMMY1
UNION ALL SELECT 'B', 'ProdB2', DATE('2020-01-12'), DATE('2020-03-14') FROM SYSIBM.SYSDUMMY1
UNION ALL SELECT 'B', 'ProdB3', DATE('2020-03-15'), DATE('2020-04-18') FROM SYSIBM.SYSDUMMY1
UNION ALL SELECT 'B', 'ProdB4', DATE('2020-04-16'), DATE('2020-05-03') FROM SYSIBM.SYSDUMMY1
)
,
*/
MYTAB_ENUM AS
(
SELECT
T.*
, ROWNUMBER() OVER (PARTITION BY PRODUCT ORDER BY START_DATE) RN
FROM MYTAB T
)
SELECT A.PRODUCT, A.END_DATE + 1 START_DT, B.START_DATE - 1 END_DT
FROM MYTAB_ENUM A
JOIN MYTAB_ENUM B ON B.PRODUCT = A.PRODUCT AND B.RN = A.RN + 1
WHERE A.END_DATE + 1 <> B.START_DATE
AND A.END_DATE < B.START_DATE;
The result is:
|PRODUCT|START_DT |END_DT |
|-------|----------|----------|
|B |2020-01-10|2020-01-11|
May be more efficient way:
WITH MYTAB2 AS
(
SELECT
T.*
, LAG(END_DATE) OVER (PARTITION BY PRODUCT ORDER BY START_DATE) END_DATE_PREV
FROM MYTAB T
)
SELECT PRODUCT, END_DATE_PREV + 1 START_DATE, START_DATE - 1 END_DATE
FROM MYTAB2
WHERE END_DATE_PREV + 1 <> START_DATE
AND END_DATE_PREV < START_DATE;

Answer 2

Thnx Mark, will try this one of these days.
Never heard of LAG in DB2 V12 for z/Os
Will read about it
Thnx