functions

I am having a hard time coming from MS SQL in terms of creating
functions. Minor things keep you from being able to create functions
that are not an issue in MS SQL so I need help in understanding why they
are an issue so that I can ultimately create the functions that I need
to. I can create and run the following function fine :

CREATE FUNCTION listSales(dt date,movie text) RETURNS SETOF ticket AS
$$
BEGIN
    select * from ticket where date = $1 and mov_num = $2;
END;
$$
LANGUAGE SQL;

however, the following will not create :
CREATE FUNCTION listSales(dt date,movie text) RETURNS SETOF ticket AS
$$
declare
    r1 int;
BEGIN
    select * from ticket where date = $1 and mov_num = $2;
END;
$$
LANGUAGE SQL;

I get a syntax error on r1 int and I have no idea why. Am I not allowed
to declare variables if they are not used?

Thanks,

Kent