(Оценка * объём предмета)/объём предмета

CREATE OR REPLACE FUNCTION student.weighted_average_grade
    (eesnimi text, perenimi text) RETURNS int AS
$$
    DECLARE
    grade student.tdeclaration%ROWTYPE;
    BEGIN
    SELECT INTO grade
    FROM student.tstudent as s
    INNER JOIN student.tdeclaration as d ON d.code_student_fk=s.code_student_pk
    INNER JOIN student.tteaching as teac ON teac.teaching_id_pk=d.teaching_id_fk
    INNER JOIN student.tsubject as sub ON teac.code_subject_fk=sub.code_subject_pk
    WHERE s.code_student_pk = d.code_student_fk;
    RETURN grade;
    END;
$$
LANGUAGE 'plpgsql';

SELECT first_name as "Имя", last_name as "Фамилия", student.weighted_average_grade(first_name, last_name) as "Средневзвешенная оценка "
FROM student.tstudent

SELECT INTO grade avg(d.grade::int)

ERROR:  invalid input syntax for integer: "(4,,,,,,,)"

ERROR:  more than one row returned by a subquery used as an expression