I have query in which returns averaged values by site and month. What I want to know is the min and max of those monthly averages by site and (the difficult part) the month when each occurred.
Here is an example:
CREATE TABLE events (
esite integer NOT NULL,
edate timestamp with time zone NOT NULL,
evalue integer NOT NULL
);
INSERT INTO events Values
(1, '2016-01-03', 11),
(2, '2016-01-05', 90),
(1, '2016-01-08', 7),
(2, '2016-01-10', 40),
(1, '2016-01-15', 12),
(1, '2016-01-18', 66),
(2, '2016-01-22', 54),
(2, '2016-02-03', 70),
(2, '2016-02-05', 56),
(1, '2016-02-08', 61),
(2, '2016-02-10', 23),
(1, '2016-02-15', 30),
(1, '2016-02-18', 15),
(1, '2016-02-22', 41);
So I'm looking for a query that returns (by site) the min and max monthly average evalues and the months at which the min and max values occurred. I can get this by using the query below:
select esite, date_trunc('month', edate) as emonth, round(avg(evalue),2) as evalue_avg from events
Group by esite, emonth
-- produces the following output:
esite | emonth | evalue_avg
2 | January, 01 2016 00:00:00 | 61.33
1 | January, 01 2016 00:00:00 | 24
1 | February, 01 2016 00:00:00 | 36.75
2 | February, 01 2016 00:00:00 | 49.67
Now for the part I'm having difficultly with, I need produce the following result: — basically the min, max values and the date(month) at which each occurred by site — (Desired Output)
-- (one row per site)
esite | avg_min | eval_avg_min_date | avg_max | eval_avg_max
1 | 24.00 | January, 01 2016 00:00:00 | 36.75 | February, 01 2016 00:00:00
2 | 49.67 |February, 01 2016 00:00:00 | 61.33 | January, 01 2016 00:00:00
I've searched around and have seen some examples using windowing and lateral joins, but I haven't been successful in getting any of them to work. This might be a pivot but since I don't have a fixed number of sites, this tends to be difficult with PostgresSQL.
I'm guessing if anyone has an easy way to do this using postgres 9.4+.
Your help would be appreciated!
Best Answer
I would recommend you do something like this - and don't go down the route of having a row per month.
What happens when you have 5 (10...15... x) years' data?
This is superior IMHO. You can always use the
CROSSTAB
table function, but I would recommend against it.This gives the result:
Note, that I have
CAST
the AVG as anINTEGER
- you can of courseROUND
this off to whatever you like. You can addEXTRACT(YEAR FROM edate) AS eyear,
after theEXTRACT(MONTH...
for more clarity and a more elegant result.