Selecting first and last values in a group

Question

I have a MySql table consisting of daily stock quotes (open, high, low, close and volume) which I\'m trying to convert into weekly data on the fly. So far, I have the follow

Answer 1

You will likely need to COALESCE function to get the first value. However, you need to make sure that days without data (weekends and holidays) have a null value for _open on those days without data.

Usage would be:

SELECT MIN(_low), MAX(_high), AVG(_volume), COALESCE(_open)
CONCAT(YEAR(_date), "-", WEEK(_date)) AS myweek
FROM mystockdata
GROUP BY myweek
ORDER BY _date;

For the last() value, I can only think of a pretty hacky solution, which would be to use GROUP_CONCAT and then string manipulation to get the last value from the list. So perhaps something like this:

SELECT MIN(_low), MAX(_high), AVG(_volume), COALESCE(_open), SUBSTRING_INDEX(GROUP_CONCAT(_close), ',', -1)
CONCAT(YEAR(_date), "-", WEEK(_date)) AS myweek
FROM mystockdata
GROUP BY myweek
ORDER BY _date;

Note you could also use the GROUP_CONCAT approach for the first item instead of coalesce if you wanted consistent looking query

SELECT MIN(_low), MAX(_high), AVG(_volume), SUBSTRING_INDEX(GROUP_CONCAT(_open), ',', 1), SUBSTRING_INDEX(GROUP_CONCAT(_close), ',', -1)
CONCAT(YEAR(_date), "-", WEEK(_date)) AS myweek
FROM mystockdata
GROUP BY myweek
ORDER BY _date;

For GROUP_CONCAT to work properly you also need to make sure the dates without values have null in _open and _close fields.