Simple upsert in Postgres using a writeable CTE

upsert.sql

create table foo (id serial primary key, data json);

with updated_foo as (
    update foo set data = '"bar"' where id = 1
    returning id
)
insert into foo (id, data)
select 1, '"foo"'
where not exists(select id from updated_foo);

-- data is inserted with value "foo"

with updated_foo as (
    update foo set data = '"bar"' where id = 1
    returning id
)
insert into foo (id, data)
select 1, '"foo"'
where not exists(select id from updated_foo);

-- data is updated with value "bar"