MySQL query for items where average price is less than X? - mysql

Question

I'm stumped with how to do the following purely in MySQL, and I've resorted to taking my result set and manipulating it in ruby afterwards, which doesn't seem ideal.
Here's the question. With a dataset of 'items' like:
id state_id price issue_date listed
1 5 450 2011 1
1 5 455 2011 1
1 5 490 2011 1
1 5 510 2012 0
1 5 525 2012 1
...
I'm trying to get something like:
SELECT * FROM items
WHERE ([some conditions], e.g. issue_date >= 2011 and listed=1)
AND state_id = 5
GROUP BY id
HAVING AVG(price) <= 500
ORDER BY price DESC
LIMIT 25
Essentially I want to grab a "group" of items whose average price fall under a certain threshold. I know that my above example "group by" and "having" are not correct since it's just going to give the AVG(price) of that one item, which doesn't really make sense. I'm just trying to illustrate my desired result.
The important thing here is I want all of the individual items in my result set, I don't just want to see one row with the average price, total, etc.
Currently I'm just doing the above query without the HAVING AVG(price) and adding up the individual items one-by-one (in ruby) until I reach the desired average. It would be really great if I could figure out how to do this in SQL. Using subqueries or something clever like joining the table onto itself are certainly acceptable solutions if they work well! Thanks!
UPDATE: In response to Tudor's answer below, here are some clarifications. There is always going to be a target quantity in addition to the target average. And we would always sort the results by price low to high, and by date.
So if we did have 10 items that were all priced at $5 and we wanted to find 5 items with an average < $6, we'd simply return the first 5 items. We wouldn't return the first one only, and we wouldn't return the first 3 grouped with the last 2. That's essentially how my code in ruby is working right now.

Answer 1

I would do almost an inverse of what Jasper provided... Start your query with your criteria to explicitly limit the few items that MAY qualify instead of getting all items and running a sub-select on each entry. Could pose as a larger performance hit... could be wrong, but here's my offering..
select
i2.*
from
( SELECT i.id
FROM items i
WHERE
i.issue_date > 2011
AND i.listed = 1
AND i.state_id = 5
GROUP BY
i.id
HAVING
AVG( i.price) <= 500 ) PreQualify
JOIN items i2
on PreQualify.id = i2.id
AND i2.issue_date > 2011
AND i2.listed = 1
AND i2.state_id = 5
order by
i2.price desc
limit
25
Not sure of the order by, especially if you wanted grouping by item... In addition, I would ensure an index on (state_id, Listed, id, issue_date)
CLARIFICATION per comments
I think I AM correct on it. Don't confuse "HAVING" clause with "WHERE". WHERE says DO or DONT include based on certain conditions. HAVING means after all the where clauses and grouping is done, the result set will "POTENTIALLY" accept the answer. THEN the HAVING is checked, and if IT STILL qualifies, includes in the result set, otherwise throws it out. Try the following from the INNER query alone... Do once WITHOUT the HAVING clause, then again WITH the HAVING clause...
SELECT i.id, avg( i.price )
FROM items i
WHERE i.issue_date > 2011
AND i.listed = 1
AND i.state_id = 5
GROUP BY
i.id
HAVING
AVG( i.price) <= 500
As you get more into writing queries, try the parts individually to see what you are getting vs what you are thinking... You'll find how / why certain things work. In addition, you are now talking in your updated question about getting multiple IDs and prices at apparent low and high range... yet you are also applying a limit. If you had 20 items, and each had 10 qualifying records, your limit of 25 would show all of the first item and 5 into the second... which is NOT what I think you want... you may want 25 of each qualified "id". That would wrap this query into yet another level...

Answer 2

What MySQL does makes perfectly sense. What you want to do does not make sense:
if you have let's say 4 items, each with price of 5 and you put HAVING AVERAGE <= 7 what you say is that the query should return ALL the permutations, like:
{1} - since item with id 1, can be a group by itself
{1,2}
{1,3}
{1,4}
{1,2,3}
{1,2,4}
...
and so on?
Your algorithm of computing the average in ruby is also not valid, if you have items with values 5, 1, 7, 10 - and seek for an average value of less than 7, element with value 10 can be returned just in a group with element of value 1. But, by your algorithm (if I understood correctly), element with value 1 is returned in the first group.
Update
What you want is something like the Knapsack problem and your approach is using some kind of Greedy Algorithm to solve it. I don't think there are straight, easy and correct ways to implement that in SQL.
After a google search, I found this article which tries to solve the knapsack problem with AI written in SQL.
By considering your item price as a weight, having the number of items and the desired average, you could compute the maximum value that can be entered in the 'knapsack' by multiplying desired_cost with number_of_items

Answer 3

I'm not entirely sure from your question, but I think this is a solution to your problem:
SELECT * FROM items
WHERE (some "conditions", e.g. issue_date > 2011 and listed=1)
AND state_id = 5
AND id IN (SELECT id
FROM items
GROUP BY id
HAVING AVG(price) <= 500)
ORDER BY price DESC
LIMIT 25
note: This is off the top of my head and I haven't done complex SQL in a while, so it might be wrong. I think this or something like it should work, though.