oracle sql query question(grouping by 2 columns)

I have a table called testgroup in my database, which is like following:

I                      J                      
---------------------- ---------------------- 
1                      a                      
1                      a                      
2                      a开发者_如何学Python 
1                      b                      
1                      c                      
2                      b      
3                      d    
2                      b 
2                      b
3                      d        

Now, I want the result as below:

I                      J                      COUNT(J) in I 
---------------------- ---------------------- ----------------------
1                      a                      2                    
2                      a                      1
1                      b                      1
1                      c                      1
2                      b                      3    
3                      d                      2            

...where count(j) in I is the number of each J related to the I.

For example: with I = 1, there are 2 a in column J, so the third column would be equal 2.

select I, J, count(*) as JinI
FROM atable
GROUP BY I, J

In fact the question is about counting I and J pairs:

select I, J, count(*) from tblName group by I, J