Question

Alright I have a table where I need to get the relevant date from a GROUP BY function…

Here is a sample of the data:
min_price, day
140000     2010-09-26
130000     2010-09-27
154991     2010-10-08
143000     2010-10-09
156470     2010-10-10

I would like this result:
130000     2010-09-27
143000     2010-10-09

Here is the SQL I use:

SELECT MIN(min_price) AS low_price, min_price, day
FROM compress
WHERE item_name = '$item'
GROUP BY EXTRACT(MONTH FROM day), EXTRACT(YEAR FROM day)

Here is what I get:
130000     2010-09-26
143000     2010-10-08

As you can see, the dates don’t match up with the min row value. I need them to match up. What SQL can I use to get that result?

Answer 1

Select C.min_price, C.day
From compress As C
Where C.item_name = '$item'
    And Exists  (
                Select 1
                From compress As C2
                Where C2.item_name = C.item_name
                    And Extract(Month From C2.day) = Extract(Month From C.day)
                    And Extract(Year From C2.day) = Extract(Year From C.day)
                Group By Extract(Month From C2.day), Extract(Year From C2.day)
                Having C.min_price = Min(C2.min_price)
                )

ADDITION

MySQL is better with joins so here’s a variant of the same query

Select C.min_price, C.day
From compress As C
    Join    (
            Select C2.item_name
                , Extract(Month From C2.day) As ItemMonth
                , Extract(Year From C2.day) As ItemYear
                , Min(C2.min_price) As MinPrice
            From compress As C2
            Group By C2.item_num, Extract(Month From C2.day), Extract(Year From C2.day)
            ) As Z
        On Z.item_name = C.item_name
            And Z.ItemMonth = Extract(Month From C.day)
            And Z.ItemYear = Extract(Year From C.day)
            And Z.MinPrice = C.min_price
Where C.item_name = '$item'

What is really slowing down any variation is the calling of Extract on every row. If you had a calendar table containing a row for each date and a column for the month and year of the date and your calendar table covered the date ranges in your data, you could then do something like:

Select C.min_price, C.day
From compress As C
    Join Calendar As Cal
        On Cal.Date = C.Day
    Join    (
            Select C3.item_name
                , Cal3.Month, Cal3.Year
                , Min(C3.min_price) As MinPrice
            From compress As C3
                Join Calendar As Cal3
                    On Cal3.Date = C3.day
            Group By C3.item_num, Cal3.Month, Cal3.Year
            ) As Z
        On Z.item_name = C.Item_name
            An Z.Month = Cal.Month
            And Z.Year = Cal.Year
            And Z.MinPrice = C.min_price
Where C.item_name = '$item'