MYSQL - Group By / Order By not working - mysql

Question

I have the following data inside a table:
id person_id item_id price
1 1 1 10
2 1 1 20
3 1 3 50
Now what I want to do is group by the item ID, select the id that has the highest value and take the price.
E.g. the sum would be: (20 + 50) and ignore the 10.
I am using the following:
SELECT SUM(`price`)
FROM
(SELECT id, person_id, item_id, price
FROM `table` tbl
INNER JOIN person p USING (person_id)
WHERE p.person_id = 1
ORDER BY id DESC) x
GROUP BY item_id
However, this query is still adding (10 + 20 + 50), which is obviously not what I need to have.
Any ideas to where I am going wrong?

Answer 1

Here is what you are trying to achieve. First you need grouping in a subquery and not in outer query. In outer query you need only sum:
SELECT SUM(`price`)
FROM
(SELECT MAX(price) as price
FROM `table` tbl
INNER JOIN person p USING (person_id)
WHERE p.person_id = 1
GROUP BY item_id) x

Answer 2

http://sqlfiddle.com/#!9/40803/5
SELECT SUM(t1.price)
FROM tbl t1
LEFT JOIN tbl t2
ON t1.person_id= t2.person_id
AND t1.item_id = t2.item_id
AND t1.id<t2.id
WHERE t1.person_id = 1
AND t2.id IS NULL;

Answer 3

I'm not sure if this is the only requirement you have. If so, try this.
SELECT SUM(price)
FROM
(SELECT MAX(price)
FROM table
WHERE person_id = 1
GROUP BY item_id)

Answer 4

First of all - you don't need the person table, because the other table already contains the person_id. So i removed it from the examples.
Your query returns a sum of prices for each item.
If you replace SELECT SUM(price) with SELECT item_id, SUM(price) you wil get
item_id SUM(`price`)
1 30
3 50
But that is not what you want. Neither is it what you wrote in the question " (10 + 20 + 50)".
Now replacing the first line with SELECT id, item_id, SUM(price) you will get one row for each item with the highest id.
id item_id price
2 1 20
3 3 50
This works because of the "undocumented feature" of MySQL, wich allows you to select columns that are not listed in the GROUP BY clause and get the first row from the subselect each group (each item in this case).
Now you only need to sum the price column in an additional outer select
SELECT SUM(price)
FROM (
SELECT id, item_id ,price
FROM (
SELECT id, person_id, item_id, price
FROM `table` tbl
WHERE tbl.person_id = 1
ORDER BY id DESC ) x
GROUP BY item_id
) y
However i do not recomend to use that "feature". While it still works on MySQL 5.6, you never know if that will work with newer versions. It already doesn't work on MariaDB.
Instead you can determite the MAX(id) for each item in an subselect, select only the rows with the determined ids and get the summed price of them.
SELECT SUM(`price`)
FROM `table` tbl
WHERE tbl.id IN (
SELECT MAX(tbl2.id)
FROM `table` tbl2
WHERE tbl2.person_id = 1
GROUP BY tbl2.item_id
)
Another solution (wich internaly does the same) is
SELECT SUM(`price`)
FROM `table` tbl
JOIN (
SELECT MAX(tbl2.id) as id
FROM `table` tbl2
WHERE tbl2.person_id = 1
GROUP BY tbl2.item_id
) x ON x.id = tbl.id
Alex's solution also works fine, if the groups (number of rows per person and item) are rather small.

Answer 5

You have used group by in main query, but it is on subquery like
SELECT id, person_id, item_id, SUM(`price`) FROM ( SELECT MAX(price) FROM `table` tbl WHERE p.person_id = 1 GROUP BY item_id ) AS x