在SQL中，将null值替换为另一个值

Here is my SQL Query

这是我的SQL查询

SELECT p.StudentID, ai.RollNo, p.FirstName, p.MiddleName, p.LastName, 
om.ExamID,  et.ExamName, om.SubjectID,
ISNULL(CONVERT(varchar(20),om.ObtainedMarksTheory), 'A') as 'ObtainedMarksTheory', 
ISNULL(CONVERT(varchar(20),om.ObtainedPracticalMarks),'A') as 'ObtainedPracticalMarks'
FROM Students.PersonalInfo p
INNER JOIN Students.AcademicCourse ac on p.StudentID = ac.StudentID
INNER JOIN Students.AcademicInfo ai on p.StudentID=ai.StudentID
LEFT OUTER JOIN Exam.ObtainedMarkEntry om on p.StudentID = om.StudentID
LEFT JOIN Exam.ExamType et   on om.ExamID = et.ExamID
WHERE ai.BatchID = '103' AND ai.SemesterID = '21' and ac.Section = '8'
SELECT p.Stu