try this:
select * from
(SELECT COURSE_ID, SNUM, to_number(GRADE)as gradenum
FROM BASE_DATA_SCORE NATURAL JOIN BASE_DATA_COURSE
WHERE COURSE_KIND =1 ) A
where A.gradenum<70
select * from
(SELECT COURSE_ID, SNUM, to_number(GRADE)as gradenum
FROM BASE_DATA_SCORE NATURAL JOIN BASE_DATA_COURSE
WHERE COURSE_KIND =1 ) A
where A.gradenum<70
(SELECT COURSE_ID, SNUM, to_number(GRADE)as gradenum
FROM BASE_DATA_SCORE NATURAL JOIN BASE_DATA_COURSE
WHERE COURSE_KIND =1 ) A
where A.gradenum<70
报告说无效的数字
SELECT COURSE_ID, SNUM, to_number(GRADE)
FROM BASE_DATA_SCORE NATURAL JOIN BASE_DATA_COURSE
WHERE COURSE_KIND =1 AND GRADE<'70' and length(grade)=length('70');
SELECT COURSE_ID, SNUM, to_number(GRADE)
FROM BASE_DATA_SCORE NATURAL JOIN BASE_DATA_COURSE
WHERE COURSE_KIND =1 AND GRADE<'70' and length(grade)<=length('70');