select within a fucntion

I`mtrying to make a select which envolves two tables with in a
function....if the query is written this way: (this is just an example,
not my query)

a := (select count(*) from xx);

it works fine, but if I type the query like this

select count(*) from xx;

it throws a message that says unexpected query in exec_stmt_execsql.
If anyone knows how to fix it, it woul be great.
Thanx