試験1z0-071-JPN トピック11 問題211 スレッド

Unused columns are a feature in Oracle that allows a column to be marked as unused without immediately dropping it from the table structure. The key points for each option are:
A: The DESCRIBE command does not display unused columns. These columns are also not visible in user_tab_cols but are visible in user_unused_col_tabs.
B: Primary key columns can be set to unused, but you must drop the primary key constraint before doing so.
C: Once a column is marked as unused, it is not possible to query it, and Oracle will not allow DML operations on that column.
D: This is true. Once a column is set to unused, you can add a new column with the same name because Oracle essentially considers the unused column as removed from the table.
E: Just like primary key columns, a foreign key column can be marked as unused, but you must drop the foreign key constraint first.
F: This is true. Unused columns retain their data until the column is dropped using the DROP UNUSED COLUMNS command.
References:
* Oracle Database SQL Language Reference, 12c Release 1 (12.1): "ALTER TABLE"
* Oracle Database Administrator's Guide, 12c Release 1 (12.1)