How to Count the number of rows within each distinct group? - mysql

Question

Given the following MySQL table:
| id | category | Hour | quantity|
| 0 | Sunday | 10 | 32 |
| 0 | Sunday | 11 | 19 |
| 0 | Sunday | 12 | 48 |
| 0 | Sunday | 19 | 7 |
| 1 | Monday | 09 | 45 |
| 1 | Monday | 10 | 17 |
| 1 | Monday | 12 | 18 |
| 2 | Tuesday | 08 | 16 |
| 2 | Tuesday | 09 | 39 |
| 2 | Tuesday | 10 | 24 |
| 2 | Tuesday | 11 | 37 |
| 2 | Tuesday | 12 | 40 |
I need to compute a fifth column which must be the division of "quantity" by the number of rows of id: for 0 there are 4 rows, for 1 3 rows, for 2 5 rows.
| id | category | Hour | quantity| avg |
| 0 | Sunday | 10 | 32 | 8 |
| 0 | Sunday | 11 | 19 | 4.75 |
| 0 | Sunday | 12 | 48 | 12 |
| 0 | Sunday | 19 | 7 | 1.75 |
| 1 | Monday | 09 | 45 | 15 |
| 1 | Monday | 10 | 17 | 5.7 |
| 1 | Monday | 12 | 18 | 6 |
| 2 | Tuesday | 08 | 16 | 3.2 |
| 2 | Tuesday | 09 | 39 | 7.8 |
| 2 | Tuesday | 10 | 24 | 4.8 |
| 2 | Tuesday | 11 | 37 | 7.4 |
| 2 | Tuesday | 12 | 40 | 8 |
How can I get the result in a MySQL query?
The first table, is the result of this query:
select id, category, Hour, count(*) as quantity
FROM table_1
GROUP by id, Hour ORDER by id, Hour;
This what I tried, in order to get the number of rows for the occurrence of each id, however I get a large number, the count of id=0 occurrences instead of id=0 rows in the previous query:
select id, Hour, count(id) as q
FROM table_1
GROUP by id
This is mySql 5.6.

Answer 1

This is really quite ugly and cumbersome, but it was the only way to get the results without having a primary key to work with:
SELECT
t.id,
t.category,
t.hour,
quantity,
ROUND(quantity/count,2) AS avg
FROM table_1 t
JOIN (SELECT
id, Hour, count(*) as quantity
FROM table_1
GROUP by id, category, Hour) AS qty
ON t.id = qty.id AND t.hour = qty.hour
JOIN (SELECT
id, count(distinct hour) as count
FROM table_1
GROUP BY id) as counts
ON t.id = counts.id
GROUP BY id, hour;
It seems to be working locally for me, at least, with guessing what your original dataset looks like.
There may well be a simpler way, however.
Edit: On second check, the 'quantity' subquery doesn't really add much that I can see, so this can be replaced with a 'count(*)', making a more optimal query:
SELECT
t.id,
t.category,
t.hour,
count(*) as quantity,
ROUND(count(*)/count,2) AS avg
FROM table_1 t
JOIN (SELECT
id, count(distinct hour) as count
FROM table_1
GROUP BY id) as counts
ON t.id = counts.id
GROUP BY id, hour;

Answer 2

You need to do the counting in a subquery that just groups by id. Join the subquery to the main query and do the division.
SELECT id, category, hour, COUNT(*) AS quantity, COUNT(*)/count AS avg
FROM table_1
JOIN (SELECT id, COUNT(DISTINCT hour) AS count
FROM table_1
GROUP BY id) AS counts
ON table_1.id = counts.id
GROUP BY table_1.id, table_1.hour
ORDER BY table_1.id, table_1.hour