Postgres 15

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create table my_table (
  a int,
  b int,
  c int,
  d int
);
  
CREATE TABLE

--  I want value in D (a+b) if C>0 and (a-b) if C<=0.
create function my_insert(a int, b int, c int)
returns void
language plpgsql
as $$
declare d integer;
begin
  case
  when c > 0 then d = a + b;
  else d = a - b;
  end case;

  insert into my_table ("a", "b", "c", "d") values (a, b, c, d);
end;
$$ 

CREATE FUNCTION

select my_insert(1, 2, 3);

my_insert

SELECT 1

select my_insert(5, 6, -3);

my_insert

SELECT 1

select * from my_table;

a
b
c
d

1
2
3
3

5
6
-3
-1

SELECT 2