What is difference for below there two sql queries - mysql

Question

select
substr(insert_date, 1, 14),
device, count(1)
from
abc.xyztable
where
insert_date >= DATE_SUB(NOW(), INTERVAL 10 DAY)
group by
device, substr(insert_date, 1, 14) ;
and then I am trying to get average of the same rows count which I got above.
SELECT
date, device, AVG(count)
FROM
(SELECT
substr(insert_date, 1, 14) AS date,
device,
COUNT(1) AS count
FROM
abc.xyztable
WHERE
insert_date >= DATE_SUB(NOW(), INTERVAL 10 DAY)
GROUP BY
device, substr(insert_date, 1, 14)) a
GROUP BY
device, date;
AS I found both queries return the same results, I tried for last 10 days data.
My purpose is to get the average rows count for last 10 days which I get from the above 1st query.

Answer 1

I'm not entirely sure what you're asking, the "difference" between the two queries is that the first one is valid but the second does not appear to be, as per HoneyBadger's comment. They also seem to be trying to achieve two different goals.
However, I think what you are trying to do is produce a query based on the data from the first query, which returns the date, device, and an average of the count column. If so, I believe the following query would calculate this:
WITH
dataset AS (
select substr(insert_date,1,14) AS theDate, device, count(*) AS
theCount
from abc.xyztable
where insert_date >=DATE_SUB(NOW(), INTERVAL 10 DAY)
group by device,substr(insert_date,1,14)
)
SELECT theDate, device, (SELECT ROUND(AVG(CAST(theCount
AS FLOAT)), 2) FROM
dataset) AS Average
FROM dataset
GROUP BY theDate, device
I have referenced the accepted answers of this question to calculate the average: How to calculate average of a column and then include it in a select query in oracle?
And this question to tidy up the query: Formatting Clear and readable SQL queries
Without having a sample of your data, or any proper context, I can't see how this would be especially useful, so if it was not what you were looking for, please edit your question and clarify exactly what you need.
EDIT: Based on what extra information you have provided, I've made a tweak to my solution to increase the precision of the average column. It now calculates the average to two decimal places. You have stated that this returns the same result as your original query, but the two queries are not formulating the same thing. If the count column is consistently the same number with little variation, the AVG function will round this, which in turn could produce results which look the same, especially if you only compare a small sample, so I have amended my answer to demonstrate this. Again, we'd all be able to help you much easier if you would provide more information, such as a sample of your data.

Answer 2

If you want an average you need to change the last GROUP BY
to get an average per device
GROUP BY device;
to get an average per date
GROUP BY date;
or remove it completely to get an average for all rows in the sub-query
Update
Below is a full example for getting the average per device
SELECT device, avg(count)
FROM (SELECT substr(insert_date,1,14) as date, device, count(1) as count
FROM abc.xyztable
WHERE insert_date >=DATE_SUB(NOW(), INTERVAL 10 DAY)
GROUP BY device,substr(insert_date,1,14)) a
GROUP BY device;