MySQL Workbench - script storing return in array and performing calculations? - mysql-workbench

Question

Firstly, this is part of my college homework.
Now that's out of the way: I need to write a query that will get the number of free apps in a DB as a percentage of the total number of apps, sorted by what category the app is in.
I can get the number of free apps and also the number of total apps by category. Now I need to find the percentage, this is where it goes a bit pear-shaped.
Here is what I have so far:
-- find total number of apps per category
select #totalAppsPerCategory := count(*), category_primary
from apps
group by category_primary;
-- find number of free apps per category
select #freeAppsPerCategory:= count(*), category_primary
from apps
where (price = 0.0)
group by category_primary;
-- find percentage of free apps per category
set #totals = #freeAppsPerCategory / #totalAppsPercategory * 100;
select #totals, category_primary
from apps
group by category_primary;
It then lists the categories but the percentage listed in each category is the exact same value.
I had initially thought to use an array, but from what I have read mySql does not seem to support arrays.
I'm a bit lost of how to proceed from here.

Answer 1

Finally figured it out. Since I had been saving the previous results in variables it seems that it was not able to calculate on a row by row basis, which is why all the percentages were identical, it was an average. So the calculation needed to be part of the query.
Here's what I came up with:
SELECT DISTINCT
category_primary,
CONCAT(FORMAT(COUNT(CASE
WHEN price = 0 THEN 1
END) / COUNT(*) * 100,
1),
'%') AS FreeAppSharePercent
FROM
apps
GROUP BY category_primary
ORDER BY FreeAppSharePercent DESC;
Then the query result is: