1. Use backup table:

A. create a table test as select * from a.

B. delete table a;

C. create table a(...- change ddd field to char(20)

D. insert select * from test

E. Drop table test

2. Use plsql dev for temporary columns.

A. add the ddd_tmp class to the table.

B. update a set of ddd_tmp = ddd.

C. Delete the ddd column

D. add ddd column characters (20)

E. update a set of ddd= ddd_tmp.

F. delete the ddd_tmp column.