How to natural join the two queries having with clause? - postgresql

Question

I have written two queries that help in finding the minimum and maximum sales quantities for different products. Now, I need to merge these two queries using natural join to output one single table.
Query 1:
with max_quant_table as
(with maxquant_table as
(select distinct month,prod as prod, sum(quant) as quant from sales group by month,prod)
select month as month,prod as MOST_POPULAR_PROD, quant as MOST_POP_TOTAL_Q
from maxquant_table)
select t2.* from
(select month, max(MOST_POP_TOTAL_Q) maxQ FROM max_quant_table group by month order by month asc)
t1 join max_quant_table t2 on t1.month = t2.month and (t2.MOST_POP_TOTAL_Q =maxQ)
Query 2:
with min_quant_table as
(with minquant_table as
(select distinct month,prod as prod, sum(quant) as quant from sales group by month,prod)
select month as month,prod as LEAST_POPULAR_PROD, quant as LEAST_POP_TOTAL_Q
from minquant_table)
select t2.* from
(select month, min(LEAST_POP_TOTAL_Q) minQ FROM min_quant_table group by month order by month asc)
t1 join min_quant_table t2 on t1.month = t2.month and (t2.LEAST_POP_TOTAL_Q = minQ)

Answer 1

You are over complicating things. You don't need to join those two query (and should really stay away from a natural join), you only need to combine them. min() and max() can be used inside the same query, there is no need to run two queries to evaluate both.
You also don't need to nest CTE definitions, you can just write one after the other.
So something like this:
with quant_table as (
select month, prod, sum(quant) as sum_q
from sales
group by month, prod
), min_max as (
select month, max(sum_q) as max_q, min(sum_q) as min_q
from quant_table
group by month
)
select t1.*
from quant_table t1
join min_max t2
on t2.month = t1.month
and t1.sum_q in (t2.min_q, t2.max_q)
order by month, prod;
The condition and t1.sum_q in (t2.min_q, t2.max_q) could also be written as and (t2.max_q = t1.sum_q or t2.min_q = t1.sum_q).
The above can further be simplified by combining group by with window functions and do the calculation of the sum, min and max in a single query:
with min_max as (
select month, prod,
sum(quant) as sum_q,
max(sum(quant)) over (partition by month) as max_q,
min(sum(quant)) over (partition by month) as min_q
from sales
group by month, prod
)
select month, prod, sum_q
from min_max
where sum_q in (max_q, min_q)
order by month, prod;