rows to colum in mysql query null value problem

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MySQL

I have 2 table category and module. category table has 2 fields id & category module has 3 fields.id,modulename & catid

category table has 2 fields :

id & category

module has 3 fields:

id,modulename & catid

category

id         |        category
-----------------------------
1          |         ABC
2          |         PQR

module

id   catid      modulename
--------------------------
1   1              A 
2   1              B
3   2              Q
4   2              P

I want to convert row to column and I'm getting a result like below:

ABC     |    PQR    
-------------------
A       |    NULL  
B       |    NULL  
NULL    |    Q
NULL    |    P

and I want result like below (without null values)

ABC      |     PQR    
-------------------
A        |      Q
B        |      P

this is my query

SELECT * FROM ( SELECT CASE WHEN category='ABC' THEN modulename ELSE '' END AS 'ABC', CASE WHEN category='PQR' THEN modulename ELSE '' END AS 'PQR' FROM category c INNER JOIN module m ON m.catid=c.id )t  

but from this query i had null values, can u tell me how to avoid this null value?