So let's say I would like to write a function called

sum_values

that sums the values in a row and then puts them at the end. Let's say I have a table that looks like this:

sql
id   value_1   value_2   result
1     600        300       0
2     400        100       0
3     100        200       0
-- etc.

The end result should look like this:

sql
id   value_1   value_2   result
1     600        300      900
2     400        100      500
3     100        200      300
-- etc.

It could be done in

sql

or

plpgsql

, I guess.

sql
create or replace function sum_values() returns void as $$
  -- 1. Select value_1 and value_2 for each row
  -- 2. Into each result insert the proper sum
$$ language sql; -- or plpgsql at this point

As a bonus, how could I make it accept the argument of a range and sum the values in that given range? Or for specific ids only? Thanks in advance, everyone!

I think it makes sense to use a generated column here. In this case you could define the table like:

sql
create table values (
  id serial primary key,
  value_1 numeric,
  value_2 numeric,
  result generated always as (value_1 + value_2) stored
);

More info about generated columns here: https://www.postgresql.org/docs/12/ddl-generated-columns.html

I don't think I want to actually create a table 🤔

Then you would do this.

sql
alter table values
add column result generated always as (value_1 + value_2) stored;