Using SQLite, how can I calculate the maximum year on year growth rate for each year? - sql

Question

I am learning about SQL and I am doing a practice exercise called World Populations SQL Practice on Codecademy. There is one table with three columns: country, population, and year. I am interested in calculating the country with the maximum year-on-year growth rate each year. (This wasn't suggested by Codecademy, I just think it's an interesting idea).
I can calculate all of the year-on-year growth rates with this query:
SELECT country,
100.0 * ((SELECT population FROM population_years AS p2
WHERE p2.year = p1.year + 1
AND p2.country = p1.country)
- population) / population AS year_on_year_growth,
year
FROM population_years AS p1
WHERE year_on_year_growth IS NOT NULL
ORDER BY year_on_year_growth;
and I can calculate the maximum year-on-year growth rate for a particular year, such as 2005, with a query such as this:
SELECT country,
100.0 * ((SELECT population FROM population_years AS p2
WHERE p2.year = p1.year + 1
AND p2.country = p1.country)
- population) / population AS year_on_year_growth,
year
FROM population_years AS p1
WHERE year = 2005
AND year_on_year_growth IS NOT NULL
ORDER BY year_on_year_growth DESC
LIMIT 1;
Using python, I can solve the problem using the first query saved as yoy_query if I do this:
yoy_result = c.execute(yoy_query).fetchall()
sorted([record for record in yoy_result if record[1] == max([row[1] for row in yoy_result if row[2] == record[2]])],key=lambda x:x[2])
and I get the desired result:
[('Montserrat', 7.34177215189872, 2000), ('Montserrat', 13.4433962264151, 2001), ('Afghanistan', 5.803891762260126, 2002), ('Montserrat', 10.467706013363028, 2003), ('Liberia', 4.7976709085316545, 2004), ('Jordan', 7.088496587486171, 2005), ('Jordan', 6.764378108744186, 2006), ('Montserrat', 12.638580931263864, 2007), ('Liberia', 4.157111008408977, 2008), ('Niger', 3.737166190281749, 2009)]
But I can't think of a way to do this using SQL. Any ideas? I think the reason it seems much easier in python is because I'm able to save the intermediate result, then run a second calculation on that.

Answer 1

You can do it with window functions LAG() and RANK():
select country, year_on_year_growth, year
from (
select *, rank() over (partition by year order by year_on_year_growth desc) as rnk
from (
select *,
100.0 * (population / lag(population) over (partition by country order by year) - 1) as year_on_year_growth
from population_years
)
)
The expression:
lag(population) over (partition by country order by year)
returns the country's population the previous year (assuming that there are no gaps between the years).
So I calculated the growth rate as:
((current year's population) / (previous year's population)) - 1

Answer 2

I guess the simplest thing to do would actually be to just use a view as follows:
CREATE VIEW yoy_growth
AS
SELECT country,
100.0 * ((SELECT population FROM population_years AS p2
WHERE p2.year = p1.year + 1
AND p2.country = p1.country)
- population) / population AS year_on_year_growth,
year
FROM population_years AS p1
WHERE year_on_year_growth IS NOT NULL
ORDER BY year_on_year_growth;
SELECT * FROM yoy_growth AS y1
WHERE year_on_year_growth = (
SELECT MAX(year_on_year_growth)
FROM yoy_growth AS y2
WHERE y1.year = y2.year
)
ORDER BY year;
That way I get the result I want, although the query does seem a little slow.