ALTER FAQS

1. What is the ALTER command in Oracle?

The ALTER statement in Oracle is used to modify the structure of an existing database object, such as a table, column, view, or index. This command allows you to add, modify, or remove parts of the schema objects.

Examples:

·        ALTER TABLE: Modify table structure (e.g., add or drop columns).

·        ALTER COLUMN: Modify a column’s attributes (e.g., data type or size).

·        ALTER INDEX: Change index settings.

2. Can I use ALTER to change table comments in Oracle?

No, you cannot use ALTER to change table or column comments directly. Instead, you use the COMMENT ON statement to modify or add comments.

For example:

COMMENT ON TABLE employees IS 'Employee information table';

3. How do I remove a comment from a table or column in Oracle?

To remove a comment from a table, column, or other object, you can set the comment to NULL:

COMMENT ON TABLE employees IS NULL;

COMMENT ON COLUMN employees.salary IS NULL;

This will remove the comments from the respective objects.

4. Can I modify the data type of a column using ALTER TABLE in Oracle?

Yes, you can modify the data type of a column in Oracle, but only in certain circumstances. For instance, you can change a column’s data type if the new type is compatible with the old type.

ALTER TABLE employees MODIFY salary NUMBER(10, 2);

However, there are some restrictions (e.g., you cannot directly change a column from VARCHAR2 to NUMBER without additional steps).

5. Can I drop a column in Oracle using ALTER TABLE?

Yes, you can drop a column from a table using the ALTER TABLE statement:

ALTER TABLE employees DROP COLUMN middle_name;

6. How can I rename a table or column in Oracle?

You can rename a table or column using ALTER TABLE with the RENAME clause.

·        Renaming a table:

ALTER TABLE old_table_name RENAME TO new_table_name;

·        Renaming a column:

ALTER TABLE employees RENAME COLUMN old_column_name TO new_column_name;

7. How can I add a new column to a table?

You can add a column to an existing table using the ALTER TABLE statement with the ADD clause:

ALTER TABLE employees ADD (email VARCHAR2(255));

8. Can I alter a table to add a primary key constraint?

Yes, you can add a primary key constraint to an existing table using the ALTER TABLE statement:

ALTER TABLE employees ADD CONSTRAINT pk_employee_id PRIMARY KEY (employee_id);

9. How do I rename a constraint in Oracle?

To rename a constraint, use the ALTER TABLE statement with the RENAME CONSTRAINT option:

ALTER TABLE employees RENAME CONSTRAINT old_constraint_name TO new_constraint_name;

10. What happens if I execute an ALTER operation that causes data inconsistency?

If an ALTER operation results in data inconsistency (e.g., trying to shrink the size of a column that has data exceeding the new size), Oracle will raise an error and will not perform the operation. You may need to manually adjust the data to fit the new column size or resolve any conflicting constraints.

11. Can I add a foreign key constraint after the table has been created?

Yes, you can add a foreign key constraint to an existing table using the ALTER TABLE statement:

ALTER TABLE orders ADD CONSTRAINT fk_customer FOREIGN KEY (customer_id) REFERENCES customers (customer_id);

12. What is the maximum length of a comment in Oracle?

Oracle allows comments to be up to 4000 characters long. This is the maximum length for comments on tables and columns.

13. Can I use ALTER to modify a view in Oracle?

Views cannot be altered in the same way as tables. If you need to change a view, you will generally need to drop and recreate it. However, you can add or remove columns in a view by modifying the underlying query.

14. Can I alter an index in Oracle?

You cannot directly alter an index in Oracle to change its structure. However, you can drop and recreate an index with the desired changes. For example:

DROP INDEX my_index;

CREATE INDEX my_index ON employees (last_name);

15. How can I track all changes made by the ALTER command?

Oracle doesn’t track ALTER commands by default. However, you can enable auditing in Oracle to capture such activities. You can set up auditing for schema changes using AUDIT commands or configure fine-grained auditing (FGA) to track specific operations.

AUDIT ALTER TABLE BY ACCESS;

16. Can I alter a sequence in Oracle?

Sequences in Oracle cannot be altered directly. If you need to modify a sequence (such as its increment or starting value), you would typically have to drop the existing sequence and create a new one:

DROP SEQUENCE seq_name;

CREATE SEQUENCE seq_name START WITH 100;

17. Can I modify an index's columns or structure?

No, you cannot modify an existing index. If you need to change the columns or structure of an index, you must drop the index and then create a new one.

DROP INDEX my_index;

CREATE INDEX my_index ON my_table (new_column);

18. Can I alter a materialized view in Oracle?

Yes, you can alter a materialized view using the ALTER MATERIALIZED VIEW statement. For example:

ALTER MATERIALIZED VIEW mv_name REFRESH COMPLETE START WITH SYSDATE NEXT SYSDATE + 1;