What exactly aore you trying to do?

The first query you posted limits the number of rows since you perform an inner join on CRS, HIS, and STU and use this result in an outer join on GRD(SN).

Since the column names are all but descriptive it's hard to figure the business logic behind it.

Just guessing I would "translate"

CN = course name

SN = student name

STU.ID = student id

HIS.PID = person id (aka student id).

If that's the case then the database design itself is inkonsistent: What would you do if there are two students with the name John Doe in one course? One received an 'A' and the other one an 'F'. How would your result set look like?

Therefore I'd like to ask you to provide the table definition together with some sample data and your expected result based on those data. For details onhow to do that please see the firstlink in my signature.