PostgreSQL, Variables - postgresql

Question

I am using PostgreSQL through Npgsql driver for windows/NET and I see that it is possible to use PL/pgSQL language through it.
So that way I can make use of variables for my calculation scripts which may look like in this example:
DO $$
DECLARE
tlist text='mylistofbills';
tcontent text='mycontentofbills';
BEGIN
CREATE TEMP TABLE tlist
(billno integer, bdate timestamp, rebate double precision)
ON COMMIT DROP;
INSERT INTO tlist
VALUES (1, '10.01.2017. 10:14:56', 10),
(2, '10.01.2017. 11:02:13', 5),
(3, '10.01.2017. 11:45:22', 0),
(4, '10.01.2017. 12:01:01', 6);
CREATE TEMP TABLE tcontent
(billno integer, rowno integer, price double precision, tax double precision)
ON COMMIT DROP;
INSERT INTO tcontent
VALUES (1, 1, 100, 19),
(1, 2, 30, 0),
(2, 1, 20, 19),
(3, 1, 18, 19),
(4, 1, 43, 0);
END $$;
SELECT s.price,
l.rebate,
s.price/100*l.rebate AS valrebate,
s.price-(s.price/100*l.rebate) AS worebate,
((s.price-(s.price/100*l.rebate))/100)*s.tax AS valtax,
s.price-(s.price/100*l.rebate)+(((s.price-(s.price/100*l.rebate))/100)*s.tax) AS finalprice
FROM tlist l, tcontent s
WHERE l.billno=s.billno;
Example is simplified (from real situation) and is suitable for pasting into PgAdmin's SQL editor.
So, now is question: Can I somehow in the body of those code, without adding new functions to server use formulas for writing more elegant and readable code?
If I would be able to add simple formulas like:
rebatec=s.price/100*l.rebate
priceworebate=s.price-rebatec
Then my code may look more readable and less error prone.
Like this:
SELECT s.price,
l.rebate,
rebatec AS valrebate,
priceworebate AS worebate,
(priceworebate/100)*s.tax AS valtax,
priceworebate+((priceworebate/100)*s.tax) AS finalprice
FROM tlist l, tcontent s
WHERE l.billno=s.billno;
If that may be possible where and how to put this formulas so it can be used in my last SELECT code?
SOLUTION:
Based on #Clodoaldo's answer which give something new to me I find a solution which I am able to understand:
SELECT s.price,
l.rebate,
rebatec AS valrebate,
priceworebate AS worebate,
priceworebate/100*s.tax AS valtax,
priceworebate+priceworebate/100*s.tax AS finalprice
FROM tlist l, tcontent s, LATERAL
(SELECT s.price/100*l.rebate AS rebatec,
s.price-s.price/100* l.rebate AS priceworebate
)sub
WHERE l.billno=s.billno;
It works and I hope it is technically correct.

Answer 1

Use lateral:
The LATERAL key word can precede a sub-SELECT FROM item. This allows the sub-SELECT to refer to columns of FROM items that appear before it in the FROM list.
select
s.price,
l.rebate,
rebatec as valrebate,
priceworebate as worebate,
priceworebate / 100 * s.tax as valtax,
priceworebate + priceworebate / 100 * s.tax as finalprice
from
tlist l
inner join
tcontent s using (billno)
cross join lateral (
select
s.price / 100 * l.rebate as rebatec,
s.price - s.price / 100 * l.rebate as priceworebate
) cjl
Use the modern join syntax.

Answer 2

You could use a subquery to define those variables:
select var1 * col3
from (
select col1 / col2 as var1
, *
from YourTable
) sub
Or alternatively a common table expression:
with cte as
(
select col1 / col2 as var1
, *
from YourTable
)
select var1 * col3
from cte