How to use a field alias in a query condition?

Question

PostgreSQL

I have a query like
select id, email, (firstname || ' ' || lastname) as fullname
from user
How to use fullname alias in Where condition?
select id, email, (firstname || ' ' || lastname) as fullname
from user
where fullname like 'abc'
Swears there is no fullname field

Answer 1

No way. In the where section, the fields from the select do not yet exist.
Duplicate the condition, wrap it in another select, make a stored function and call it in where and in select.