ORA-01440 : column to be modified must be empty to decrease precision or scale

oracle number data type format :  number( precision, scale)

Precision 4, scale 2   : 99.99
Precision 10, scale 0 : 9999999999
Precision 8, scale 3   : 99999.999
Precision 5, scale -3  : 99999000

How to modify  column number(10)  to number(10,2)

Column number(10,2) has  2 decimal places of precision.
The 2 digits of precision being added to the column would presumably come at the cost of precision to the left of the decimal place.
Hence, ORA-01440 error  occurs

you have to increase the precision (10)  by 2
” alter table test_table1  modify (  a(12,2)  ) ;  ”

To modify  column number(6,2)  to number( 6,3 )

you have to increase the precision (6)  by 1
” alter table test_table2  modify ( 7,3)  ) ;  “