Referring to a different column in PostgreSQL column - postgresql

Question

Consider the following image
If you want to get a result row containing all steps to get the length of the non-labeled sides, you can do the following:
SELECT
5 AS a, --side 1, triangle 1
7 AS b, --side 2, triangle 1
(5*5) AS a2, --a^2
(7*7) AS b2, --b^2
(5*5)+(7*7) AS c2, --a^2 * b^2 = c^2
SQRT((5*5)+(7*7)) AS c, --√c2 = c
19 AS d, --side 1, triangle 2
24 AS e, --side 2 triangle 2
(19*19) AS d2, --d^2
(24*24) AS e2, --e^2
(19*19)+(24*24) AS f2, --d^2 * e^2 = f^2
SQRT((19*19)+(24*24)) AS f, --√f2 = f
(5*5)+(7*7)+(19*19)+(24*24) AS g2, --c^2 * f^2 = g^2
SQRT((5*5)+(7*7)+(19*19)+(24*24)) AS g --√g2 = g
However, that is CLEARLY very ugly. I'd like to use column substitution, like:
SELECT
5 AS a, --side 1, triangle 1
7 AS b, --side 2, triangle 1
(a*a) AS a2, --a^2
(b*b) AS b2, --b^2
a2+b2 AS c2, --a^2 * b^2 = c^2
SQRT(c2) AS c, --√c2 = c
19 AS d, --side 1, triangle 2
24 AS e, --side 2 triangle 2
(d*d) AS d2, --d^2
(e*e) AS e2, --e^2
d+e AS f2, --d^2 * e^2 = f^2
SQRT(f2) AS f, --√f2 = f
c2+f2 AS g2, --c^2 * f^2 = g^2
SQRT(g2) AS g --√g2 = g
Is there any easy way to do this?
PS Please don't explain how this is a ridiculous use of SQL, I know THAT! This was just the simplest way that I could reduce my problem to be understood. In my scenario, it is much more complex calculations with variables coming from many joined tables, that the results need to be inserted into a summary table with a very rigid structure. Currently, I'm bringing the results out to Node doing the calculations and inserting the data, but that is very VERY slow especially since I to go through the network to get to the database server.

Answer 1

This can be done using common table expressions:
with base_vars (a,b,d,e) as (
values (5),(7),(19),(24)
), var2 (a2, b2, d2, e2) as (
select a*a, b*b, d*d*, e*e
from base_vars,
), var3 (c2, c, f2, f) as (
select a2+b2, SQRT(a2+b2), d+e, sqrt(d+e)
from var2, base_vars
), var3 (g2, g) as (
select c2+f2, sqrt(c2+f2)
from var3
)
select sqrt(g)
from var3;
I am not 100% if I got all variables right, but I think you get the idea.
Another option would be to put that into a PL/pgSQL function.

Answer 2

lateral is a bit shorter than CTEs since it is not necessary to refer to a previous CTE. And the planner can not join the CTEs and the main query into a single plan.
with t (a,b,d,e) as (values (5,7,19,24))
select c, f, sqrt(c2 + f * f)
from
t
cross join lateral
(select a * a, b * b, d * d, e * e) t1 (a2, b2, d2, e2)
cross join lateral
(select a2 + b2, d2 + e2) t2 (c2, f2)
cross join lateral
(select sqrt(c2), sqrt(f2)) t3 (c, f)
;
c | f | sqrt
------------------+------------------+------------------
8.60232526704263 | 30.6104557300279 | 31.7962261911693