I want to assign rank following with some condition - mysql

Question

I want to assign the rank to 'drug_name' as per the order of 'svcdate' for each 'patient_id' in a dataset. (here, to describe the issue I'm only showing one patient_id in the image)
select patient_id
,svcdate
,drug_name
,dense_rank() over(partition by patient_id order by first_date) as rank
from (
select *
,first_value(svcdate) over (
partition by patient_id, drug_name
order by svcdate) as first_date
from table
)
order by 1,2;
With this query I'm getting the following output,
Although, I want something like this (as shown in image below)
Please help me understand what I'm missing out in the query and how to address this issue.
Thanks!!

Answer 1

using this CTE for the data:
with data(patient_id, svcdate, drug_name) as (
select * from values
(110, '2018-08-09'::date, 'TRANEXAMIC ACID'),
(110, '2020-05-28'::date, 'TAKHZYRO'),
(110, '2020-06-10'::date, 'ICATIBANT'),
(110, '2020-06-24'::date, 'TAKHZYRO'),
(110, '2020-07-22'::date, 'TAKHZYRO'),
(110, '2020-07-24'::date, 'ICATIBANT'),
(110, '2020-08-31'::date, 'ICATIBANT'),
(110, '2020-08-31'::date, 'TAKHZYRO')
)
And using CONDITONAL_CHANGE_EVENT gives you what you want
select patient_id
,svcdate
,drug_name
,CONDITIONAL_CHANGE_EVENT( drug_name ) OVER (
PARTITION BY patient_id ORDER BY svcdate )+1 as rank
from data
order by 1,2;
gives:
PATIENT_ID
SVCDATE
DRUG_NAME
RANK
110
2018-08-09
TRANEXAMIC ACID
1
110
2020-05-28
TAKHZYRO
2
110
2020-06-10
ICATIBANT
3
110
2020-06-24
TAKHZYRO
4
110
2020-07-22
TAKHZYRO
4
110
2020-07-24
ICATIBANT
5
110
2020-08-31
ICATIBANT
5
110
2020-08-31
TAKHZYRO
6

Answer 2

We can try to use LAG window function in the subquery to get each previous drug_name, then compare by condition aggregate window function to make rank column.
select patient_id
,svcdate
,drug_name
,SUM(CASE WHEN prev_drug_name <> drug_name THEN 1 ELSE 0 END) over(partition by patient_id order by first_date) as rank
from (
select *,LAG(drug_name) OVER(partition by patient_id ORDER BY svcdate) prev_drug_name
from table
)
order by 1,2;