Grouping multiple column operations in 1 table scan - sql

Question

I need to translate SAS code (PROC SQL) to (postgres) SQL, especially the calculated keyword in SAS that allow a variable defined in the query to be re-used directly in the same query for another variable computation:
SELECT
id,
sum( case
when (sales > 0) then 1
when (sales = 0) then 0
else -1
end) as pre_freq,
(case
when calculated pre_freq > 0 then calculated pre_freq
else 1
end) as freq
FROM my_table
GROUP BY id
This is not possible (AFAIK) in SQL, so I need to break down each step of the computation.
I was wondering what was the best option, knowing that, from my understanding, it is better to have more computations and fewer table scans, i.e. make as much as computation during a scan, instead of multiple table scans with a small computation steps.
In this particular exemple I could use:
SELECT
id
, greatest(1, sum( case
when (sales > 0) then 1
when (sales = 0) then 0
else -1
end) as freq
FROM
my_table
GROUP BY id
or:
SELECT
id
, (case when sum(case
when (sales > 0) then 1
when (sales < 0) then -1
else 0
end) > 0 then sum(case
when (sales > 0) then 1
when (sales < 0) then -1
else 0
end) else 1 end) as freq
FROM
my_table
GROUP BY id
... which is starting to be hard to read...
Is there anyway to define a variable for a snippet of SQL code that will be repeated?
More generally speaking that this illustration, was is the best (most efficient) approach?

Answer 1

calculated is a nice feature of proc sql. However, you cannot re-use aliases in databases in general (this is not a Postgres-specific limitation). A simple method is to use a subquery or CTE:
select id, pre_freq,
(case when pre_freq > 0 then pre_freq
else 1
end) as freq
from (select id,
sum(case when (sales > 0) then 1
when (sales = 0) then 0
else -1
end) as pre_freq,
from my_table t
group by id
) t;
However, the simplest solution is to use sign():
select id, sum(sign(sales)) as pre_freq,
greatest(sum(sign(sales)), 1) as freq
from my_table t
group by id;
Note: This is slightly different. It basically ignores NULL values. If you really need to treat NULL as -1, then use coalesce().