MySQL: Group by Austral year? - mysql

Question

Let's say we have a simple table like so
DATE VALUE
--------------------
2013-01-01 23
2013-06-12 34
2013-07-22 788
2013-12-16 234
2014-01-23 56
2014-04-19 88
2014-09-23 7987
2014-11-05 66
2015-02-17 987
2015-05-05 896
Now let's suppose we want to extract the mean values grouped by year, we would use
SELECT AVG(`VALUE`) FROM tablename GROUP BY YEAR(`DATE`)
However, what if we define the year not from January to December, but rather from July to June (which makes quite a bit of sense for a lot of data if you're living in the Southern hemisphere).
I am sure there must be an easy way to achieve a July YEAR to June YEAR+1 grouping. It just seems to elude me.
-- EDIT --
Awesome! Two good solutions arrived within minutes of posting this question, that both deserved to be accepted. I accepted Roland's solution because of its brevity, but Leo Tara's reply made me aware that I forgot about outputting the associated austral year. So based on both answers I came up with this solution which does exactly what I was asking for:
SELECT
YEAR(`DATE` - INTERVAL 6 MONTH) as `australyear`,
AVG(`VALUE`)
FROM tablename
GROUP BY YEAR(`DATE` - INTERVAL 6 MONTH)

Answer 1

Just add 6 month to the date when grouping. I don't know the exact syntax in case of mysql, sorry.
Pseudocode:
SELECT AVG(`VALUE`) FROM tablename GROUP BY YEAR(`DATE` + '6 MONTHS');

Answer 2

Try this:
SELECT AVG(`VALUE`)
FROM tablename
GROUP BY
CASE
WHEN MONTH('DATE') BETWEEN 1 AND 6 THEN YEAR('DATE') - 1
ELSE YEAR(`DATE`)
END
If you want to show year on field list you must rewrite your query in this way:
SELECT
CASE
WHEN MONTH('DATE') BETWEEN 1 AND 6 THEN YEAR('DATE') - 1
ELSE YEAR(`DATE`)
END, AVG(`VALUE`)
FROM tablename
GROUP BY
CASE
WHEN MONTH('DATE') BETWEEN 1 AND 6 THEN YEAR('DATE') - 1
ELSE YEAR(`DATE`)
END
Test on Sql Fiddle