Postgres 14

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

version

PostgreSQL 14.7 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 8.5.0 20210514 (Red Hat 8.5.0-16), 64-bit

SELECT 1

create table main_table( product_id    integer generated always as identity
                       , product_stock integer 
                       , product_name  text
                       ) ;
                       
create table update_table(product_id  integer
                         , product_stock integer 
                         ); 
                       
insert into main_table (product_stock, product_name)
   values (100,'ABC'), (200, 'DEF');
   
select * 
  from main_table 
 order by product_id;
  
insert into update_table(product_id, product_stock)
    values (1,75), (3,400), (2,500);

select * 
  from update_table; 

CREATE TABLE

CREATE TABLE

INSERT 0 2

product_id
product_stock
product_name

1
100
ABC

2
200
DEF

SELECT 2

INSERT 0 3

product_id
product_stock

1
75

3
400

2
500

SELECT 3

update main_table mt  
   set product_stock = ut.product_stock
  from update_table ut
 where ut.product_id = mt.product_id;

UPDATE 2

select * 
  from main_table
 order by product_id;

product_id
product_stock
product_name

1
75
ABC

2
500
DEF

SELECT 2