How to create a view that show last 4 quarters data? - mysql

Question

My environment is 5.1.72-community.
I have a table named production, like:
id, person_id, num, p_year, p_quarter
1, 1, 43, 2018, 2
2, 1, 92, 2018, 1
3, 1, 108, 2017, 4
4, 2, 41, 2018, 2
...
By the time of '2018-04-14', we know quarter=2, and year=2018. I define quarter_1 is current quarter of current year, and quarter_2 is previous quarter, ans so on. Now I want to create a view like:
person_id, num_quarter_1, num_quarter_2, num_quarter_3, num_quarter_4
1, 43, 92, 108, 0
2, 41, 0, 0, 0
As that, if there no num in that quarter, fill 0.
How to create that view please?

Answer 1

Hmmm. If I interpret "last four quarters" as "the last four quarters that are in the data", then here is one method.
First, you can get the four quarters by doing:
select distinct p_year, p_quarter
from production p
order by p_year desc, p_quarter desc
limit 4;
You can enumerate them using variables:
select p_year, p_quarter, (#rn := #rn + 1) as enum
from (select distinct p_year, p_quarter
from production p
order by p_year desc, p_quarter desc
limit 4
) p cross join
(select #rn := 0) params
Then, you can use this in a query to pivot the data:
select p.person_id,
sum(case when seqnum = 1 then num else 0 end) as num_quarter_1,
sum(case when seqnum = 2 then num else 0 end) as num_quarter_2,
sum(case when seqnum = 3 then num else 0 end) as num_quarter_3,
sum(case when seqnum = 4 then num else 0 end) as num_quarter_4
from production p join
(select p_year, p_quarter, (#rn := #rn + 1) as seqnum
from (select distinct p_year, p_quarter
from production p
order by p_year desc, p_quarter desc
limit 4
) p cross join
(select #rn := 0) params
) yq
using (p_year, p_quarter)
group by p.person_id;
EDIT:
If you are defining the quarters based on the current quarter, you can do something quite similar:
select p.person_id,
sum(case when seqnum = 1 then num else 0 end) as num_quarter_1,
sum(case when seqnum = 2 then num else 0 end) as num_quarter_2,
sum(case when seqnum = 3 then num else 0 end) as num_quarter_3,
sum(case when seqnum = 4 then num else 0 end) as num_quarter_4
from production p join
(select year(curdate()) as p_year, quarter(curdate()) as p_quarter, 1 as seqnum union all
select year(curdate() - interval 1 quarter) as p_year, month(curdate() - interval 1 quarter) as p_quarter, 2 as seqnum union all
select year(curdate() - interval 2 quarter) as p_year, month(curdate() - interval 2 quarter) as p_quarter, 3 as seqnum union all
select year(curdate() - interval 2 quarter) as p_year, month(curdate() - interval 3 quarter) as p_quarter, 4 as seqnum
) yq
using (p_year, p_quarter)
group by p.person_id;
There are other approaches, such as:
select person_id,
sum(case when year(curdate()) = p_year and quarter(curdate()) = p_quarter
then num else 0
end) as num_quarter_1,
sum(case when year(curdate() - interval 1 quarter) = p_year and quarter(curdate() - interval 1 quarter) = p_quarter
then num else 0
end) as num_quarter_2,
sum(case when year(curdate() - interval 2 quarter) = p_year and quarter(curdate() - interval 2 quarter) = p_quarter
then num else 0
end) as num_quarter_3,
sum(case when year(curdate() - interval 3 quarter) = p_year and quarter(curdate() - interval 3 quarter) = p_quarter
then num else 0
end) as num_quarter_4
from production p
group by person_id;

Answer 2

SELECT
person_id,
SUM((p_quarter = 1) * num) as num_quarter_1,
SUM((p_quarter = 2) * num) as num_quarter_2,
SUM((p_quarter = 3) * num) as num_quarter_3,
SUM((p_quarter = 4) * num) as num_quarter_4
FROM production
WHERE p_year = 2018
GROUP BY person_id;