Postgres 9.5

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select version();

version

PostgreSQL 9.5.25 on x86_64-pc-linux-musl, compiled by gcc (Alpine 11.2.1_git20220219) 11.2.1 20220219, 64-bit

SELECT 1

create table stock (sku text primary key, qty int);
insert into stock values ('apple',10);

CREATE TABLE

INSERT 0 1

-- INSERT ... ON CONFLICT arrived in PostgreSQL 9.5
insert into stock values ('apple',5),('pear',3)
on conflict (sku) do update set qty = stock.qty + excluded.qty;

INSERT 0 2

select * from stock order by sku;

sku
qty

apple
15

pear
3

SELECT 2