Avoid Null values in Coalesce - sql

Question

I have used coalesce to produce results but got duplicate rows with Null values.
Sample tables as below:
Table - Student
ID Student Subject id Subject Grade id
100 Peter 200 Chinese 201
101 Mary 300 English 302
102 Sam 400 Maths 403
103 John 900 Music
Chinese Table
Subject id Grade id Grade
200 201 Good
200 202 Average
200 203 Poor
English Table
Subject id Grade id Grade
300 301 Good
300 302 Average
300 303 Poor
Maths Table
Subject id Grade id Grade
400 401 Good
400 402 Average
400 403 Poor
Select Id, Name,
Coalesce (chinese.grade, english.grade, maths.grade)
from Student
Left join Chinese On student.id = Chinese.subjectId AND student.gradeId = Chinese.gradeId
Left join English On student.id = English.subjectId AND student.gradeId = Enlgish.gradeId
Left join Maths On student.id = Maths.subjectId AND student.gradeId = Maths.gradeId
Result
ID Student Subject Grade
100 Peter Chinese Good
100 Peter Chinese NULL
101 Mary English Average
101 Mary English NULL
102 Sam Maths Poor
102 Sam Maths NULL
103 John Music NULL
I am not sure where the duplicated row of Null values came from, just want to know how can I avoid giving null values by using coalesce?

Answer 1

Answering the "just want to know how can I avoid giving null values by using coalesce?" part of the question only, since the origin of the duplicate records requires more information than was provided:
COALESCE returns the first non-null value in its argument list, or NULL if all arguments are NULL.
So Select Id, Name, Coalesce (chinesetable.grade, englishtable.grade, mathstable.grade) ... will still give you a NULL grade if all of chinesetable.grade, englishtable.grade, and mathstable.grade are NULL. You need to decide what you want to do in that situation. You could either skip those rows:
Select * From
(Select Id, Name, Coalesce (chinesetable.grade, englishtable.grade, mathstable.grade) As Grade from Student where id = id...) As grades_with_nulls
Where Grade Is Not Null;
Or provide a default value as the last argument to Coalesce:
Select Id, Name, Coalesce (chinesetable.grade, englishtable.grade, mathstable.grade, 'None') from Student where id = id...

Answer 2

Use:
select s.id,s.name,s.[subject],
coalesce(coalesce(coalesce(c.grade,e.grade,c.grade),e.grade, coalesce(c.grade,e.grade,c.grade)),m.grade,coalesce(coalesce(c.grade,e.grade,c.grade),e.grade, coalesce(c.grade,e.grade,c.grade))) as grade
from student s
left join chinese c on s.subjectid = c.subjectid and c.gradeid = s.gradeid
left join english e on e.subjectid = s.subjectid and e.gradeid = s.gradeid
left join maths m on m.subjectid = s.subjectid and m.gradeid = s.gradeid
result
id--name----subject--grade
100 Peter Chinese Good
101 Mary English Average
102 Sam Maths Poor
103 John Music NULL