SQL Select counter when values change - sql

Question

I need to make a counter for everytime a value changes in time.
I have this table:
Date | Quantity
2017-02-01 | 10000
2017-02-02 | 20000
2017-02-03 | 20000
2017-02-04 | 20000
2017-02-05 | 10000
2017-02-06 | 10000
I want to make something like this:
Date | Quantity | Counter
2017-02-01 | 10000 | 1
2017-02-02 | 20000 | 2
2017-02-03 | 20000 | 2
2017-02-04 | 20000 | 2
2017-02-05 | 10000 | 3
2017-02-06 | 10000 | 3
I tried using dense_rank and other functions but I couldn't make it look like that because it would give the same Counter number when the quantity is 10000.
Is what I'm asking even possible?
Thank you!

Answer 1

A simple method is to use lag() and a cumulative sum:
select t.date, t.quantity,
sum(case when quantity = prev_quantity then 0 else 1 end) over (order by date) as counter
from (select t.*, lag(quantity) over (order by date) as prev_quantity
from t
) t;
These are ANSI standard functions and available in most databases.

Answer 2

Simple solution for Oracle 12 and above only, using the MATCH_RECOGNIZE clause:
with
test_data ( dt, quantity ) as (
select date '2017-02-01', 10000 from dual union all
select date '2017-02-02', 20000 from dual union all
select date '2017-02-03', 20000 from dual union all
select date '2017-02-04', 20000 from dual union all
select date '2017-02-05', 10000 from dual union all
select date '2017-02-06', 10000 from dual
)
-- end of test data, for illustration only; WITH clause is NOT part of the query
-- solution (SQL query) begins BELOW THIS LINE
select dt, quantity, mn as counter
from test_data
match_recognize (
order by dt
measures match_number() as mn
all rows per match
pattern ( a b* )
define b as b.quantity = a.quantity
)
;
DT QUANTITY COUNTER
---------- ---------- ----------
2017-02-01 10000 1
2017-02-02 20000 2
2017-02-03 20000 2
2017-02-04 20000 2
2017-02-05 10000 3
2017-02-06 10000 3
6 rows selected.