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?