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The instructor is correct. Once you set a column to unused, there is no way to make it used again. This feature is just intended to allow you to hide one or more columns from a large table in preparation for dropping them later when you have a larger downtime window. It is intended to be just as irreversible as dropping the column.
Once you user SET UNUSED COLUMN then you cannot get back the column again, it is treated as a dropped column. Moreover you can add a new column with that name.
Any time you can drop the SET UNUSED COLUMNS with the following command.
ALTER TABLE table_name DROP UNUSED COLUMNS;
i am new to database.Since you're new:
Is there any option of to make an unused column to reusable again?
Try to find your answers in the Oracle documentation as well, as it explains clearly how 'Oracle things' work, with examples and you can do quick searches on keywords of your interest.
You should bookmark: http://tahiti.oracle.com and start reading the concepts and fundamentals when you've got some spare time.
assuming you're on 10g
1) [http://www.orafaq.com/node/968] although this link is talking about the table BUT
can i get back the total column from BIN (after/before removing the column)?
this link is saying exactly the same until and unless this part:
"...Also, if you mark a column of datatype LONG as UNUSED, you cannot add
another LONG column to the table until you actually drop the unused LONG
---cant i add a new col with same name and same data types?
1. no. a table will go to the recycle bin afte a drop table command.
2. this is correct, there can be only one long column in a table, but after dropping it
you can use the same column name:
SQL> create table p(l long); Table created. SQL> alter table p add l2 long; alter table p add l2 long * ERROR at line 1: ORA-01754: a table may contain only one column of type LONG SQL> alter table p add l2 number; Table altered. SQL> alter table p set unused column l; Table altered. SQL> alter table p add l clob; Table altered.