Re: Extract elements from JSON array and return them as concatenated string

Yes, here x is the alias for the record, not for the json field. So you need to write the query like 

select string_agg(x->>'letter', ' ')
from (
   select jsonb_array_elements(tiles) x
   from words_moves
   where gid=656 and action='play'
   order by played desc limit 5
) y;