Postgres 12

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
select version();


version


PostgreSQL 12.13 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 8.5.0 20210514 (Red Hat 8.5.0-15), 64-bit



SELECT 1


create table balances
(
    username   varchar(255)  not null,
    currency   varchar(255)  not null,
    balance    numeric       null,
    constraint balances_pkey primary key (username, currency)
); 

insert into balances(username, currency,balance)
 values ('A','Aus$', 50), ('B','Aus$',200);

CREATE TABLE



INSERT 0 2


create or replace 
function merge_balance(
         p_username varchar  
       , p_currency varchar  
       , p_to_add   numeric
       )
  returns  void
  language sql
as $$
    insert into balances as bal (username, currency, balance)
    values (p_username, p_currency, p_to_add)
        on conflict (username, currency) 
        do update 
           set balance = bal.balance + excluded.balance;
$$; 

CREATE FUNCTION


-- test 
select * from balances
order by username;


username
currency
balance


A
Aus$
50

B
Aus$
200



SELECT 2


do $$
begin
   perform merge_balance('A','Aus$',75);
   perform merge_balance('C','Aus$',300);
end ;
$$;

DO


select * from balances
order by username;


username
currency
balance


A
Aus$
125

B
Aus$
200

C
Aus$
300



SELECT 3