Write a Postgres Get or Create SQL Query

Question

I want to write a single Postgres SQL statement that says look for a user with color X and brightness Y. If that user exists, return all of its row data. If not, create a new

Answer 1

with sel as (
    select color, brightness, size, age
    from mytable
    where color = 'X' and brightness = 'Y'
), ins as (
    insert into mytable (color, brightness, size, age)
    select 'X', 'Y', 6.2, 40
    where not exists (
        select 1 from sel
    )
    returning color, brightness, size, age
)
select color, brightness, size, age
from ins
union
select color, brightness, size, age
from sel