NUMBER(18,6)
show numwidth;
UPDATE xx SET xa ='123456789012.123456' where xb='111'; 
xa
-------------------
123456789000
设置为15位
set numwidth 15; 

xa
------------------
123456789012.12

或者

TO_CHAR(xa,'FM099999999999.099999')
xa
-------------
123456789012.123456