问题
I have a sql (Presto) table that contains data about different kinds of bird species. The table contains information about how often the different species have been observed in a park over different months. Some of the species have not been observed in certain months, but measurements were only performed on 07-9-19, 08-10-19, and 05-11-19. A sample of the data would look like this
Species Period Total (Cumulative) Observations (TCO)
Bird1 07-9-19 33
Bird1 08-10-19 45
Bird1 05-11-19 60
Bird2 07-9-10 20
Bird3 .... ....
Bird3 .... ....
.... .... ....
What I would like is to add a new column to this table that captures incremental change between two days of measurement for each species. Based on the example sample I would like to produce the following result:
Species Period TCO Change
Bird1 07-9-19 33 33
Bird1 08-10-19 45 12
Bird1 05-11-19 60 15
Bird2 07-9-10 20 20
Bird3 08-10-19 5 5
Bird3 05-11-19 24 19
.... .... ....
I think I've found a way to do this, but it is quite complicated and involves creating a new table. I was wondering if there was are simpler solution to my problem (as it is slightly simpler than the problem in the link).
回答1:
You can use window function lag() to recover the previous tco for the same species, period-wise:
select
t.*,
tco - lag(tco, 1, 0) over(partition by species order by period) change
from mytable
The three-argument form of lag lets you specify a default value instead of null: as per your expected result, you want 0 in that case.
来源：https://stackoverflow.com/questions/58493773/how-to-show-incremental-changes-in-sql