SQL Query to find the Row with first change of data - sql

Question

UniqueId
ITEM
DATE
1
A
2022-01-01
2
A
2022-01-02
3
B
2022-01-03
4
B
2022-01-04
5
A
2022-01-05
6
A
2022-01-06
7
B
2022-01-07
8
B
2022-01-08
9
A
2022-01-09
10
A
2022-01-10
11
A
2022-01-11
I have above table where the item is changing from A to B and then B to A (etc).
The the most recent item in the table based on the date is A (the last row).
I need to find the date on which this last item (A) was started to be in effect.
So in this case the item A was in effect from 2022-01-09 onwards (UniqueId 9).
How can I find the UniqueId or the date of item A, where it got changed to be in effect (Row 9)?
Thank you.

Answer 1

with data as (
select *,
last_value(item) over (order by "date") as last_item,
lag(item) over (order by "date") as prev_item
from T
)
select
max(case when item = last_item and item <> prev_item then "date" end) as max_date
from data;
or
with data as (
select *,
case when item <> lag(item) over (order by "date")
and item = last_value(item) over (order by "date")
then 1 end as flag
from T
)
select max("date") as last_transition_date
from data
where flag = 1;
https://dbfiddle.uk/?rdbms=sqlserver_2019&fiddle=bd5f6398c0167d74c26a67fafac5225e
Supposing you need all the data:
with data as (
select *,
case when item <> lag(item) over (order by "date")
and item = last_value(item) over (order by "date")
then 1 end as flag
from T
)
select *,
max(case when flag = 1 then "date" end) over () as last_transition_date
from data;

Answer 2

Getting a flag using a comparison of current item with previous item in time, using LAG() is indeed the way.
But it's absolutely sufficient to get the highest date and highest unique (as both are sorted ascending together) where the obtained flag is 1:
WITH
-- your input
indata(UniqueId,ITEM,DATE) AS (
SELECT 1,'A',DATE '2022-01-01'
UNION ALL SELECT 2,'A',DATE '2022-01-02'
UNION ALL SELECT 3,'B',DATE '2022-01-03'
UNION ALL SELECT 4,'B',DATE '2022-01-04'
UNION ALL SELECT 5,'A',DATE '2022-01-05'
UNION ALL SELECT 6,'A',DATE '2022-01-06'
UNION ALL SELECT 7,'B',DATE '2022-01-07'
UNION ALL SELECT 8,'B',DATE '2022-01-08'
UNION ALL SELECT 9,'A',DATE '2022-01-09'
UNION ALL SELECT 10,'A',DATE '2022-01-10'
UNION ALL SELECT 11,'A',DATE '2022-01-11'
)
-- real query starts here; replace following comma with "WITH"
,
w_change_ind AS (
SELECT
*
, CASE WHEN LAG(item) OVER(ORDER BY date) <> item
THEN 1
ELSE 0
END AS chg_ind
FROM indata
)
SELECT
MAX(uniqueid) AS uqid
, MAX(date) AS dt
FROM w_change_ind
WHERE chg_ind=1
;
-- out uqid | dt
-- out ------+------------
-- out 9 | 2022-01-09

Answer 3

Based on your description, this is one way to do what you want.
select top 1 * from table1
where item ='A'
order by uniqueid desc
If this is not what you want, then you will have to provide additional information.