Question

Is there a purely SQL way to do this?

I have a table apples: id, price, and another apple_info: apple_id, color

For each row in apples, I want to add a corresponding row in apple_info, if it does not already exist. Can’t find any examples of this.

Answer 1

Try this:

insert into apple_info (apple_id, color)
select a.id, 'some_color'
  from apples a
  left outer join apple_info ai
    on (a.id = ai.apple_id)
 where ai.apple_id is null;

You will insert a row on apple_info with non existent apple_id and a fixed value for color. I guess this is what you want.