Table: sales_data

customer_name sales_date     amount
------------- ----------     ------
C1            12-JAN-2021      1500
C1            25-JAN-2021      2000
C1            12-MAR-2021       800
C1            12-APR-2021       900
C2            12-FEB-2021      1500
C2            12-JUL-2021      1800


Sample output:

Customer sales_month total_amount
-------- ----------- ------------
C1       JANUARY             3500
C1       FEBRUARY               0
C1       MARCH                800
C1       APRIL                900
C1       MAY                    0
..
..
C1       DECEMBER               0
C2       JANUARY                0  
C2       FEBRUARY            1500
C2       MARCH                  0
C2       APRIL                  0
C2       MAY                    0
C2       JUNE                   0
C2       JULY                1800
..
..
C2       DECEMBER               0