Getting wrong sum when using group by with inner join

问题

With reference to this question (How to get the sum in a joined table when using group by - getting wrong results) I have two tables orders and order_items. I need to group the results by days. But I also need to get the sum of energy_used for each day from another table. When I try that using a join, I get wrong order_sum for each day (they are not being summed up). Not sure what I am doing wrong.

I would like to get for each day

the sum of order_items.energy_used for all orders created that day
the sum of orders.order_sum for all orders created that day
the created_at and order_sum that correspond to the latest order created on that day

Here is my orders table

+----+-----------+---------+---------------------+
| id | order_sum | user_id | created_at          |
+----+-----------+---------+---------------------+
| 1  | 25.13     | 7       | 2020-01-25 09:13:00 |
| 2  | 10.00     | 7       | 2020-01-25 15:23:00 |
| 3  | 14.00     | 5       | 2020-01-26 10:14:00 |
| 4  | 35.00     | 1       | 2020-01-27 11:13:00 |
+----+-----------+---------+---------------------+

And here is my order_items table

+----+----------+-------------+---------------------+
| id | order_id | energy_used | created_at          |
+----+----------+-------------+---------------------+
| 1  | 1        | 65          | 2020-01-25 09:13:00 |
| 2  | 1        | 12          | 2020-01-25 09:13:00 |
| 3  | 2        | 70          | 2020-01-26 10:14:00 |
| 4  | 2        | 5           | 2020-01-26 10:14:00 |
| 5  | 3        | 0           | 2020-01-27 11:13:00 |
+----+----------+-------------+---------------------+

And this is the desired result that I am trying to achieve

+---------------+-----------------+-------------------+---------------------+----------------+
| date_of_month | total_order_sum | total_energy_used | last_order_date     | last_order_sum |
+---------------+-----------------+-------------------+---------------------+----------------+
| 2020-01-25    | 35.13           | 77                | 2020-01-25 09:13:00 | 25.13          |
| 2020-01-26    | 14.00           | 75                | 2020-01-26 10:14:00 | 14.00          |
| 2020-01-27    | 35.00           | 0                 | 2020-01-27 11:13:00 | 35.00          |
+---------------+-----------------+-------------------+---------------------+----------------+

And here is the query that I have tried but I'm getting wrong results, the order_sum is not being calculated correctly. It is showing the same as last_order_sum

select 
    date(o.created_at) date_of_month,
    i.total_energy_used,
    o.created_at last_order_date,
    o.order_sum last_order_sum,
    sum(order_sum) as total_order_sum
from orders o
inner join (
    select date(o1.created_at) date_of_month, sum(i1.energy_used) total_energy_used
    from orders o1
    inner join order_items i1 on o1.id = i1.order_id
    group by date(o1.created_at)
) i on i.date_of_month = date(o.created_at)
where o.created_at = (
    select max(o1.created_at)
    from orders o1
    where date(o1.created_at) = date(o.created_at)
)

Here is a fiddle: https://dbfiddle.uk/?rdbms=mysql_5.6&fiddle=92b8cc2920ad9f7a7cdd56bded5a3bf2

回答1:

Always join tables together on their relationships (in this case orders.id with order_items.order_id) and then group. to avoid duplicating order_sums for multiple order_items when joining, first group order_items by order_id.

select 
    date(o.created_at) date_of_month,
    sum(i.total_energy_used),
    max(o.created_at),
    sum(order_sum) as total_order_sum
from orders o
inner join (
    select order_id, sum(total_energy_used) total_energy_used
    from order_items i
    group by order_id
) i on o.id = i.order_id
group by date(o.created_at)

from this point onwards you can do a join again on orders with max(o.created_at) to get the order_sum of the last order. moral of the story: keep an eye on your granularity.

回答2:

Your problem is that you are selecting from orders, where you actually want an aggregate of orders by date. So select from two aggregtating subqueries that you join. Only problem is last_order_sum, which we can select in a further subquery, once we know the last order date.

select 
  order_date,
  o.total_order_sum,
  oi.total_energy_used,
  o.last_order_date,
  (
    select order_sum
    from orders last_order
    where lastorder.created_at = o.last_order_date
  ) as last_order_sum
from 
(
  select
    date(created_at) as order_date,
    sum(order_sum) as total_order_sum,
    max(created_date) as last_order_date
  from orders
  group by date(created_at)
) o
inner join 
(
  select
    date(created_at) as order_date,
    sum(energy_used) as total_energy_used
  from order_items
  group by date(created_at)
) oi using(order_date)
order by order_date;

回答3:

What you are asking and what you show us in output are not correlated. Assuming that it is a typo:

select so.dtDay as date_of_month, so.order_sum as total_order_sum,
   eu.energy_used as total_energy_used,
   o.created_at as last_order_date,
   o.order_sum as last_order_sum
from (
select left(created_at,10) as dtDay, sum(order_sum) as order_sum, max(id) as last_insert_id
from orders
group by left(created_at,10)
order by created_at
) so
inner join orders o on o.id = so.last_insert_id
left join (select left(created_at,10) as dtDay, sum(energy_used) as energy_used
from order_items
group by left(created_at,10)) eu on so.dtDay = eu.dtDay;

DBFiddle

来源：https://stackoverflow.com/questions/59933093/getting-wrong-sum-when-using-group-by-with-inner-join

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