What does "s.a" mean in this generate_series() query

Question

I'm trying this for returning business days, but I don´t understand s.a and s(a) in this query:

SELECT dia 
FROM (
    SELECT ('2012-08-01'::date + s.a * '1 day'::interval) AS dia
    FROM generate_series(0, '2012-08-31'::date - '2012-08-01'::date, 1) AS s(a)
) foo
WHERE EXTRACT(DOW FROM dia) BETWEEN 1 AND 5
EXCEPT
SELECT feriado_data FROM teste.feriado;

Could someone explain it for me?

Answer 1

s in s(a) is a table alias given to the set returned by generate_series and the a in s(a) is the alias given to its only column.

Refer to table expression docs and scroll to "Another form of table aliasing gives temporary names to the columns of the table" for full details.