Evaluate this ALTER TABLE statement: (Choose the best answer.)
Which statement is true?
A. After executing the ALTER TABLE command, a new column called ORDER_DATE can be added to the ORDERS table.
B. The ORDER_DATE column must be empty for the ALTER TABLE command to execute successfully.
C. ROLLBACK can be used to restore the ORDER_DATE column.
D. The DESCRIBE command would still display the ORDER_DATE column.
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any statement is correct.
A – correct,
If you are concerned about the length of time it could take to drop column data from all of the rows in a large table, you can use the ALTER TABLE…SET UNUSED statement. This statement marks one or more columns as unused, but does not actually remove the target column data or restore the disk space occupied by these columns. However, a column that is marked as unused is not displayed in queries or data dictionary views, and its name is removed so that a new column can reuse that name. All constraints, indexes, and statistics defined on the column are also removed.
A – correct,
If you are concerned about the length of time it could take to drop column data from all of the rows in a large table, you can use the ALTER TABLE…SET UNUSED statement. This statement marks one or more columns as unused, but does not actually remove the target column data or restore the disk space occupied by these columns. However, a column that is marked as unused is not displayed in queries or data dictionary views, and its name is removed so that a new column can reuse that name. All constraints, indexes, and statistics defined on the column are also removed.
No A, but D is correct !
ALTER TABLE…SET UNUSED statement marks one or more columns as unused, but does not actually remove the target column data or restore the disk space occupied by these columns.
A – correct,
Just test out with ‘desc orders’ , you would know it doesnt exist in the table structure anymore