NOT NULL FAQS

 1. What is a NOT NULL constraint in Oracle?

The NOT NULL constraint in Oracle ensures that a column cannot have NULL values. It enforces that every row in the table must have a value for that column.

2. How do I apply a NOT NULL constraint on a column in Oracle?

You can apply the NOT NULL constraint during table creation or modification using the following syntax:

• When creating a table:

CREATE TABLE table_name (

    column_name datatype NOT NULL

);

• When modifying an existing table:

ALTER TABLE table_name

MODIFY column_name datatype NOT NULL;

3. Can I apply a NOT NULL constraint on an existing column that contains NULL values?

No, you cannot add a NOT NULL constraint to a column if it already contains NULL values. You would need to update those NULL values first to a valid value before applying the constraint.

4. Does a PRIMARY KEY automatically imply a NOT NULL constraint?

Yes, a PRIMARY KEY constraint automatically enforces the NOT NULL constraint. Since primary keys must be unique and cannot have NULL values, Oracle automatically ensures that a column defined as a primary key will never accept NULLs.

5. What happens if I try to insert a NULL value into a NOT NULL column?

Oracle will raise an error if you attempt to insert a NULL value into a column that has the NOT NULL constraint. The insert statement will fail, and no data will be inserted into the table.

6. Can a column with a NOT NULL constraint have a default value?

Yes, a column defined with a NOT NULL constraint can also have a default value. If no value is provided during an insert, the default value will be used instead of NULL. The NOT NULL constraint ensures that a value will always be present.

Example:

CREATE TABLE employees (

    employee_id NUMBER PRIMARY KEY,

    first_name VARCHAR2(50),

    last_name VARCHAR2(50) DEFAULT 'Doe' NOT NULL

);

7. Can I remove a NOT NULL constraint from a column in Oracle?

Yes, you can remove the NOT NULL constraint using the ALTER TABLE statement. The syntax is as follows:

ALTER TABLE table_name

MODIFY column_name datatype NULL;

This will allow the column to accept NULL values.

8. How can I check which columns have a NOT NULL constraint in a table?

You can query the USER_TAB_COLUMNS view to check for columns that have a NOT NULL constraint:

SELECT column_name

FROM user_tab_columns

WHERE table_name = 'your_table_name'

AND nullable = 'N';

9. Can I have a NOT NULL constraint on a column with a FOREIGN KEY constraint?

Yes, you can have a NOT NULL constraint on a column with a FOREIGN KEY constraint. However, whether the foreign key column should be NOT NULL depends on the business logic. If it is essential for every record to have a reference, then NOT NULL would be appropriate. If the foreign key is optional, then the column may need to allow NULL values.

10. Does NOT NULL apply to every kind of column?

NOT NULL can be applied to any column in a table, including integer, character, date, etc. However, it should be used where NULL values are not acceptable for logical or business reasons.

11. Can I insert a NULL value in a column if it doesn't have the NOT NULL constraint?

Yes, you can insert NULL values into a column as long as the column does not have the NOT NULL constraint. This is useful for optional data fields where NULL indicates that the value is not yet provided.

12. Can a NOT NULL column be indexed?

Yes, columns with a NOT NULL constraint can be indexed, and it often improves query performance. Indexing columns that are not nullable ensures that queries can efficiently filter out NULLs when necessary.

13. How does the NOT NULL constraint affect performance?

The performance impact of the NOT NULL constraint is minimal. However, it can improve the performance of queries by reducing the need to check for NULL values, making data retrieval more efficient in certain cases.

14. Can I define a NOT NULL constraint on a column after data is inserted?

Yes, but only if the column does not contain any NULL values. If the column has NULL values, you must first update the NULL entries to valid values before applying the NOT NULL constraint.

15. What is the difference between a NOT NULL constraint and a UNIQUE constraint?

• A NOT NULL constraint ensures that a column cannot have NULL values.
• A UNIQUE constraint ensures that all the values in the column are unique (no duplicates), but it does not prevent NULL values. Multiple NULLs are allowed in a column with a UNIQUE constraint.

16. Can I add a NOT NULL constraint to a column that already has data?

Yes, as long as the existing data in the column does not contain NULL values. If there are NULL values, you must either delete or update them before applying the NOT NULL constraint.