Oracle Database Discussions

I have a table called cust_file, his table consists of a lot of columns (one of these columns called cus_tax) and have a lot of data,

I use oracle 11g, I want to change the default value of the column cus_tax to be equal 1,

I wrote

ALTER TABLE cust_file MODIFY(cus_tax DEFAULT 1);

table altered

but after I inserted new data to test the operation, I found that the new record has a value = null for the column cus_tax

then  I tested using the following query

select data_default from all_tab_columns where table_name='CUST_FILE' and column_name='CUS_TAX';

no rows selected

so please help me to Change the default value of the column cus_tax.

Thanks for any help.