calculate percentage with subqueries - mysql

Question

I'm trying to calculate a percentage in my SQL query.
This is what I have right now:
SELECT
DATE(processed_at) AS day,
(
SELECT
COUNT(*) FROM return_items
WHERE return_id IN (SELECT id FROM returns WHERE DATE(processed_at) = day)
) as products_returned,
COUNT(*) as return_count,
(
SELECT
COUNT(*) as co_returns
FROM returns
WHERE return_method = 'mondial_relais'
AND DATE(processed_at) = day
) as return_rate_mr
FROM returns
WHERE MONTH(processed_at) = 10
AND YEAR(processed_at) = 2011
GROUP BY day;
Basically I need the return_rate_mr to be a percentage value.
I tried doing something like return_rate_mr * 100 / return_count as perc_value but this doesn't work. (I don't actually need the current return_rate_mr value, just the percentage.
Any ideas?

Answer 1

Assuming your original query returns the desired results, you can wrap it as a subquery:
SELECT
day,
return_rate_mr * 100 / return_count as perc_value,
... any other columns ...
FROM
( ... your original query here ...) as myalias;
Basically, the subquery creates a result set where the columns are renamed. Then, the outer query is free to use those new column names.

Answer 2

Are you looking for this?
SELECT
`day`,
`products_returned`,
(`return_rate_mr` * 100) / `return_count` AS `percentage`
FROM (
SELECT
DATE(processed_at) AS day,
(
SELECT
COUNT(*) FROM return_items
WHERE return_id IN (SELECT id FROM returns WHERE DATE(processed_at) = day)
) as products_returned,
COUNT(*) as return_count,
(
SELECT
COUNT(*) as co_returns
FROM returns
WHERE return_method = 'mondial_relais'
AND DATE(processed_at) = day
) as return_rate_mr
FROM returns
WHERE MONTH(processed_at) = 10
AND YEAR(processed_at) = 2011
GROUP BY day) AS `ss`

Answer 3

Did you try something like:
SELECT (`return_rate_mr` * 100 ) / `return_count` as "yourValue", OthersFields
FROM SELECT
DATE(processed_at) AS day,
(
SELECT
COUNT(*) FROM return_items
WHERE return_id IN (SELECT id FROM returns WHERE DATE(processed_at) = day)
) as products_returned,
COUNT(*) as return_count,
(
SELECT
COUNT(*) as co_returns
FROM returns
WHERE return_method = 'mondial_relais'
AND DATE(processed_at) = day
) as return_rate_mr
FROM returns
WHERE MONTH(processed_at) = 10
AND YEAR(processed_at) = 2011
GROUP BY day;
Hope this helps

Answer 4

Try:
SELECT DATE(processed_at) AS day,
count(distinct id) as products_returned,
COUNT(*) as return_count,
100* sum(case return_method when 'mondial_relais' then 1 end) / COUNT(*)
as return_perc_mr
FROM returns
WHERE MONTH(processed_at) = 10
AND YEAR(processed_at) = 2011
GROUP BY day;
I suspect that products_returned should be counting distinct item_id values (or something similar), but this should duplicate the logic in the original query.