How to modify data type in Oracle with existing rows in table

Question

How can I change DATA TYPE of a column from number to varchar2 without deleting the table data?

Answer 1

You can't.

You can, however, create a new column with the new data type, migrate the data, drop the old column, and rename the new column. Something like

ALTER TABLE table_name
  ADD( new_column_name varchar2(10) );

UPDATE table_name
   SET new_column_name = to_char(old_column_name, <>);

ALTER TABLE table_name
 DROP COLUMN old_column_name;

ALTER TABLE table_name
 RENAME COLUMN new_column_name TO old_coulumn_name;

If you have code that depends on the position of the column in the table (which you really shouldn't have), you could rename the table and create a view on the table with the original name of the table that exposes the columns in the order your code expects until you can fix that buggy code.