在SQL中的group by语句之后添加等级列

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I am using a data set in SQL like the first three columns. What I want to add is another column called rank. That will provide me the rank of that class in that specific year. I honestly don't know where to start. Does anyone know how to solve the issue? Thanks!

class   year   grade   rank
A       2015   21      3
A       2016   22      2
A       2017   23      1
B       2015   27      1
B       2016   25      2
B       2017   24      3
C       2015   22      3
C       2016   28      2
C       2017   29      1