How to get data from previos year? - sql

Question

Here my base sample
I need get data from previous period with lag in Hello table
Could you help me?
+------+--------+------+-------+
| Year | Animal | Plus | Hello |
+-------+------+--------+------+
| 2 | Cat | 3 | |
| 2 | Dog | 4 | |
| 2 | Mouse | 5 | |
| 3 | Cat | 5 | 3 |
| 3 | Dog | 6 | 4 |
| 3 | Mouse | 6 | 5 |
| 3 | Horse | 6 | |
| 3 | Pig | 6 | |
| 3 | Goose | 6 | |
| 4 | Cat | | 5 |
| 4 | Dog | | 6 |
| 4 | Mouse | | 6 |
| 4 | Horse | | 6 |
| 4 | Pig | | 6 |
+-------+------+--------+------+

Answer 1

You are looking for LAG. This function looks into previous rows.
select
place, year, animal, plus,
lag(plus) over (partition by animal order by year) as hello
from mytable
order by year, animal;
The "previous" row is the closest previous one, i.e. if for ' Goose' there are rows for year 3 and 5 and none for year 4, then year 3 would be considered the previous row for year 5 and LAG would show that value.
If you really want the adjacent previous year, i.e. year - 1, then you can select this year as follows:
select
place, year, animal, plus,
(
select plus
from mytable prev_year
where prev_year.animal = mytable.animal
and prev_year.year = mytable.year - 1)
) as hello
from mytable
order by year, animal;
Same thing with an outer join:
select
t.place, t.year, t.animal, t.plus, prev_year.plus as hello
from mytable t
left join mytable prev_year on prev_year.animal = t.animal
and prev_year.year = t.year - 1
order by t.year, t.animal;