How to perform computation on a temporary table using MySQL? - mysql

Question

I am trying to get the average of a column after doing a division grouped by weeks.
Here is my original table
id
week
amount
div
1
5
50
5
2
5
40
3
3
4
35
3
4
4
60
10
5
6
70
9
First I want to SUM them with group by weeks, and divide the amount/div
week
amount
div
amount/div
5
90
8
11.25
4
95
13
7.30
6
70
9
7.77
Now to get the average of amount/div which is (11.25+7.3+7.77)/3 = 8.77
I just want to get the 8.77
Here is what I tried:
SELECT AVG(amount/ div) from mytable GROUP BY week
but I didn't get the desired result of 8.77

Answer 1

Sample DB made.
http://sqlfiddle.com/#!9/6c7fa7/9
Used FLOOR to match your values 11.25, 7.3, 7.77, usual ROUND(...,2) got 1 cent difference.
CREATE TABLE stats (
id INT PRIMARY KEY AUTO_INCREMENT,
week INT,
amount INT,
divs INT
);
INSERT INTO stats ( week, amount, divs ) VALUES ( 5, 50, 5 );
INSERT INTO stats ( week, amount, divs ) VALUES ( 5, 40, 3 );
INSERT INTO stats ( week, amount, divs ) VALUES ( 4, 35, 3 );
INSERT INTO stats ( week, amount, divs ) VALUES ( 4, 60, 10 );
INSERT INTO stats ( week, amount, divs ) VALUES ( 6, 70, 9 );
SELECT ROUND(AVG(amount_divs), 2)
FROM (
SELECT FLOOR(100* SUM(amount)/SUM(divs))/100 AS amount_divs
FROM stats
GROUP BY week
) x
If you run following query
SELECT AVG(amount/divs)
FROM stats
It has different meaning, it is the average of all records (5 values), not the 3 after SUM