Combine two queries with monthly average - sql

Question

I need to put together the results of these two queries into a single return with the following structure:
"date", avg(selic."Taxa"), avg(titulos."puVenda")
Partial structure of tables:
selic
"dtFechamento" date,
"pTaxa" real
titulos
"dtTitulo" date,
"puVenda" real,
"nomeTitulo" character(30)
Query table selic:
select to_char("dtFechamento", 'YYYY-MM') as data, avg("pTaxa")
from "selic"
group by data
order by data
Query table titulos:
select to_char("dtTitulo", 'YYYY-MM') as data, avg("puVenda")
from "titulos"
where "nomeTitulo" = 'LFT010321'
group by data
order by data
I tried a subquery, but it returned the fields next to each other and can not muster.
select *
from (select to_char("dtFechamento", 'YYYY-MM') as data, avg("pTaxa")
from "selic"
group by data
order by data) as selic,
(select to_char("dtTitulo", 'YYYY-MM') as data, avg("puVenda")
from "titulos"
where "nomeTitulo" = 'LFT010321'
group by data
order by data) as LFT010321;

Answer 1

Assuming you want to return one row per month where either of your two queries returns a row. And pad missing values from the other query with NULL.
Use a FULL [OUTER] JOIN:
SELECT to_char(mon, 'YYYY-MM') AS data, s.avg_taxa, t.avg_venda
FROM (
SELECT date_trunc('month', "dtFechamento") AS mon, avg("pTaxa") AS avg_taxa
FROM selic
GROUP BY 1
) s
FULL JOIN (
SELECT date_trunc('month', "dtTitulo") AS mon, avg("puVenda") AS avg_venda
FROM titulos
WHERE "nomeTitulo" = 'LFT010321'
GROUP BY 1
) t USING (mon)
ORDER BY mon;
It is substantially faster to join after aggregating than before (fewer join operations).
It is also faster to GROUP BY, JOIN and ORDER on timestamp values than on a text rendition. Typically also cleaner and less error prone (although text is unambiguous in this particular case). That's why I use date_trunc() instead of to_char() on lower levels.
If the format for the month is not important, you can just return the timestamp value. Else you can format any way you like after you are done processing.
Similar case with more explanation:
PostgreSQL merge two queries with COUNT and GROUP BY in each

Answer 2

This should get what you need. The inner "PQ" (PreQuery) does a union all between each possible date, but also adds a flag column to identify which average it was associated with. Each part is grouped by date. So now, the outer query will AT MOST have 2 records for a given date... one for tax, the other be Venda. So now you dont need any full outer join, nor need to build some dynamic calendar data basis to get the details for all possible dates.
So, it is possible for only a Tax average OR a Venda average OR BOTH.
SELECT
PQ.Data,
SUM( CASE when PQ.SumType = 'T' then PQ.TypeAvg else 0 end ) as AvgTax,
SUM( CASE when PQ.SumType = 'V' then PQ.TypeAvg else 0 end ) as AvgVenda
from
( select
to_char( dtFechamento, 'YYYY-MM') as data,
'T' as sumtype,
avg( pTaxa ) as TypeAvg
from
selic
group by
to_char( dtFechamento, 'YYYY-MM') as data
UNION ALL
select
to_char( dtTitulo, 'YYYY-MM') as data,
'V' as sumType,
avg( puVenda ) as TypeAvg
from
titulos
where
nomeTitulo = 'LFT010321'
group by
to_char( dtTitulo, 'YYYY-MM') ) PQ
group by
PQ.Data
order by
PQ.Data