Oracle 18c

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create table customers (
  id int,
  name varchar2(100),
  company varchar2(200),
  state varchar2(2),
  
  constraint pk_customers primary key (id)
)
create table orders (
  id int,
  customer_id int,
  order_date date,
  due_date date,
  some_id int,
  
  constraint pk_orders primary key (id)
)
insert into customers(id, name, company, state)
values (1, 'Some name', 'Some company', 'NY')
1 rows affected

insert all
  when cust_exists = 0
  then
    into customers (id, name, company, state)
    values (cust_id, cust_name, company, state)

  when 1 = 1
  then
    into orders (id, customer_id, order_date, due_date, some_id)
    values(order_id, cust_id, order_date, due_date, some_id)
select
  1 as order_id,
  1 as cust_id,
  'Some other name' as cust_name,
  'Company' as company,
  'NY' as state,
  date '2021-09-28' as order_date,
  date '2021-10-03' as due_date,
  100 as some_id,
  nvl(max(1), 0) as cust_exists
from customers
where id = 1
1 rows affected

insert all
  when cust_exists = 0
  then
    into customers (id, name, company, state)
    values (cust_id, cust_name, company, state)

  when 1 = 1
  then
    into orders (id, customer_id, order_date, due_date, some_id)
    values(order_id, cust_id, order_date, due_date, some_id)
select
  2 as order_id,
  2 as cust_id,
  'Some other name' as cust_name,
  'Company' as company,
  'NY' as state,
  date '2021-09-28' as order_date,
  date '2021-10-03' as due_date,
  100 as some_id,
  nvl(max(1), 0) as cust_exists
from customers
where id = 2
2 rows affected

insert /*+
  ignore_row_on_dupkey_index(customers(id))
*/
into customers (id, name, company, state)
values (2, 'Name', 'Comp', 'NY')
insert into orders (id, customer_id, order_date, due_date, some_id)
values (3, 2, date '2021-09-30', date '2021-10-07', 5)
1 rows affected

select *
from customers


ID
NAME
COMPANY
STATE


1
Some name
Some company
NY

2
Some other name
Company
NY