ROW_NUMBER() Based on Dates - sql

Question

I have the following data:
test_date
2018-07-01
2018-07-02
...
2019-06-30
2019-07-01
2019-07-02
...
2020-06-30
2020-07-01
I want to increment a row_number value every time right(test_date,5) = '07-01' so that my final result looks like this:
test_date row_num
2018-07-01 1
2018-07-02 1
... 1
2019-06-30 1
2019-07-01 2
2019-07-02 2
... 2
2020-06-30 2
2020-07-01 3
I tried doing something like this:
, ROW_NUMBER() OVER (
PARTITION BY CASE WHEN RIGHT(a.[test_date],5) = '07-01' THEN 1 ELSE 0 END
ORDER BY a.[test_date]
) AS [test2]
But that did not work out for me.
Any suggestions?

Answer 1

Use datepart to identify the correct date, and then add 1 to a sum every time it changes (assuming there will never be more than 1 row per date).
declare #Test table (test_date date);
insert into #Test (test_date)
values
('2018-07-01'),
('2018-07-02'),
('2019-06-30'),
('2019-07-01'),
('2019-07-02'),
('2020-06-30'),
('2020-07-01');
select *
, sum(case when datepart(month,test_date) = 7 and datepart(day,test_date) = 1 then 1 else 0 end) over (order by test_date asc) row_num
from #Test
order by test_date asc;
Returns:
test_date
row_num
2018-07-01
1
2018-07-02
1
2019-06-30
1
2019-07-01
2
2019-07-02
2
2020-06-30
2
2020-07-01
3

Answer 2

You can do it with DENSE_RANK() window function if you subtract 6 months from your dates:
SELECT test_date,
DENSE_RANK() OVER (ORDER BY YEAR(DATEADD(month, -6, test_date))) row_num
FROM tablename
See the demo.
Results:
test_date | row_num
---------- | -------
2018-07-01 | 1
2018-07-02 | 1
2019-06-30 | 1
2019-07-01 | 2
2019-07-02 | 2
2020-06-30 | 2
2020-07-01 | 3

Answer 3

build a running total based on month=7 and day=2
declare #Test table (mykey int,test_date date);
insert into #Test (mykey,test_date)
values
(1,'2018-07-01'),
(2,'2018-07-02'),
(3,'2019-06-30'),
(4,'2019-07-01'),
(5,'2019-07-02'),
(6,'2020-06-30'),
(7,'2020-07-01');
select mykey,test_date,
sum(case when DatePart(Month,test_date)=7 and DatePart(Day,test_date)=2 then 1 else 0 end) over (order by mykey) RunningTotal from #Test
order by mykey