Postgresql - How to get value from last record of each month - sql

Question

I have a view like this:
Year | Month | Week | Category | Value |
2017 | 1 | 1 | A | 1
2017 | 1 | 1 | B | 2
2017 | 1 | 1 | C | 3
2017 | 1 | 2 | A | 4
2017 | 1 | 2 | B | 5
2017 | 1 | 2 | C | 6
2017 | 1 | 3 | A | 7
2017 | 1 | 3 | B | 8
2017 | 1 | 3 | C | 9
2017 | 1 | 4 | A | 10
2017 | 1 | 4 | B | 11
2017 | 1 | 4 | C | 12
2017 | 2 | 5 | A | 1
2017 | 2 | 5 | B | 2
2017 | 2 | 5 | C | 3
2017 | 2 | 6 | A | 4
2017 | 2 | 6 | B | 5
2017 | 2 | 6 | C | 6
2017 | 2 | 7 | A | 7
2017 | 2 | 7 | B | 8
2017 | 2 | 7 | C | 9
2017 | 2 | 8 | A | 10
2017 | 2 | 8 | B | 11
2017 | 2 | 8 | C | 12
And I need to make a new view which needs to show average of value column (let's call it avg_val) and the value from the max week of the month (max_val_of_month). Ex: max week of january is 4, so the value of category A is 10. Or something like this to be clear:
Year | Month | Category | avg_val | max_val_of_month
2017 | 1 | A | 5.5 | 10
2017 | 1 | B | 6.5 | 11
2017 | 1 | C | 7.5 | 12
2017 | 2 | A | 5.5 | 10
2017 | 2 | B | 6.5 | 11
2017 | 2 | C | 7.5 | 12
I have use window function, over partition by year, month, category to get the avg value. But how can I get the value of the max week of each month?

Answer 1

Assuming that you need a month average and a value for the max week not the max value per month
SELECT year, month, category, avg_val, value max_week_val
FROM (
SELECT *,
AVG(value) OVER (PARTITION BY year, month, category) avg_val,
ROW_NUMBER() OVER (PARTITION BY year, month, category ORDER BY week DESC) rn
FROM view1
) q
WHERE rn = 1
ORDER BY year, month, category
or more verbose version without window functions
SELECT q.year, q.month, q.category, q.avg_val, v.value max_week_val
FROM (
SELECT year, month, category, avg(value) avg_val, MAX(week) max_week
FROM view1
GROUP BY year, month, category
) q JOIN view1 v
ON q.year = v.year
AND q.month = v.month
AND q.category = v.category
AND q.max_week = v.week
ORDER BY year, month, category
Here is a dbfiddle demo for both queries

Answer 2

And here is my NEW version.
My thanks to #peterm for pointing me about the prior false value of val_from_max_week_of_month. So, I corrected it:
SELECT
a.Year,
a.Month,
a.Category,
max(a.Week) AS max_week,
AVG(a.Value) AS avg_val,
(
SELECT b.Value
FROM decades AS b
WHERE
b.Year = a.Year AND
b.Month = a.Month AND
b.Week = max(a.Week) AND
b.Category = a.Category
) AS val_from_max_week_of_month
FROM decades AS a
GROUP BY
a.Year,
a.Month,
a.Category
;
The new results:

Answer 3

First, you might need to check, how do you handle the first week in January. If 1st of January are not a Monday, there are several interpretations & not every one of them will fit the solutions here. You'll either need to use:
the ISO week concept, ie. the week column should hold the ISO week & the year column should hold the ISO year (week-year, rather). Note: in this concept, 1st of January actually sometimes belongs to the previous year
use your own concept, where the first week of the year is "split" into two if 1st of January is not a Monday.
Note: the solutions below will not work if (in your table) the first week of January can be 52 or 53.
Given that avg_val is just a simple aggregation, while max_val_of_month can be calculated with typical greatest-n-per-group queries. It has a lot of possible solutions in PostgreSQL, with varying performance. Fortunately, your query will naturally have an easily determined selectivity: you'll always need (approx.) a quarter of your data.
Usual winners (in performance) are:
(These are not surprise though, as these 2 should perform more and more as you need more portion of the original data.)
array_agg() with order by variant:
select year, month, category, avg(value) avg_val,
(array_agg(value order by week desc))[1] max_val_of_month
from table_name
group by year, month, category;
distinct on variant:
select distinct on (year, month, category) year, month, category,
avg(value) over (partition by year, month, category) avg_val,
value max_val_of_month
from table_name
order by year, month, category, week desc;
The pure window function variant is not that bad either:
row_number() variant:
select year, month, category, avg_val, max_val_of_month
from (select year, month, category, value max_val_of_month,
avg(value) over (partition by year, month, category) avg_val,
row_number() over (partition by year, month, category order by week desc) rn
from table_name) w
where rn = 1;
But the LATERAL variant is only viable with an index:
LATERAL variant:
create index idx_table_name_year_month_category_week_desc
on table_name(year, month, category, week desc);
select year, month, category,
avg(value) avg_val,
max_val_of_month
from table_name t
cross join lateral (select value max_val_of_month
from table_name
where (year, month, category) = (t.year, t.month, t.category)
order by week desc
limit 1) m
group by year, month, category, max_val_of_month;
But most of the solutions above can actually utilize this index, not just this last one.
Without the index: http://rextester.com/WNEL86809
With the index: http://rextester.com/TYUA52054

Answer 4

with data (yr, mnth, wk, cat, val) as
(
-- begin test data
select 2017 , 1 , 1 , 'A' , 1 from dual union all
select 2017 , 1 , 1 , 'B' , 2 from dual union all
select 2017 , 1 , 1 , 'C' , 3 from dual union all
select 2017 , 1 , 2 , 'A' , 4 from dual union all
select 2017 , 1 , 2 , 'B' , 5 from dual union all
select 2017 , 1 , 2 , 'C' , 6 from dual union all
select 2017 , 1 , 3 , 'A' , 7 from dual union all
select 2017 , 1 , 3 , 'B' , 8 from dual union all
select 2017 , 1 , 3 , 'C' , 9 from dual union all
select 2017 , 1 , 4 , 'A' , 10 from dual union all
select 2017 , 1 , 4 , 'B' , 11 from dual union all
select 2017 , 1 , 4 , 'C' , 12 from dual union all
select 2017 , 2 , 5 , 'A' , 1 from dual union all
select 2017 , 2 , 5 , 'B' , 2 from dual union all
select 2017 , 2 , 5 , 'C' , 3 from dual union all
select 2017 , 2 , 6 , 'A' , 4 from dual union all
select 2017 , 2 , 6 , 'B' , 5 from dual union all
select 2017 , 2 , 6 , 'C' , 6 from dual union all
select 2017 , 2 , 7 , 'A' , 7 from dual union all
select 2017 , 2 , 8 , 'A' , 10 from dual union all
select 2017 , 2 , 8 , 'B' , 11 from dual union all
select 2017 , 2 , 7 , 'B' , 8 from dual union all
select 2017 , 2 , 7 , 'C' , 9 from dual union all
select 2018 , 2 , 7 , 'C' , 9 from dual union all
select 2017 , 2 , 8 , 'C' , 12 from dual
-- end test data
)
select * from
(
select
-- data.*: all columns of the data table
data.*,
-- avrg: partition by a combination of year,month and category to work out -
-- the avg for each category in a month of a year
avg(val) over (partition by yr, mnth, cat) avrg,
-- mwk: partition by year and month to work out -
-- the max week of a month in a year
max(wk) over (partition by yr, mnth) mwk
from
data
)
-- as OP's interest is in the max week of each month of a year, -
-- "wk" column value is matched against
-- the derived column "mwk"
where wk = mwk
order by yr,mnth,cat;