Group by and order by can use aliases

In PostgreSQL the alias of a selected expression can be used in the group by and order by clauses instead of repeating the expression.

For example, this:

select left(lower(email), 1) as first_letter, count(*) as num
from users
group by left(lower(email), 1)
order by count(*) desc;

Can be replaced with this:

select left(lower(email), 1) as first_letter, count(*) as num
from users
group by first_letter
order by num desc;