NOT NULL

The NOT NULL constraint is one of the fundamental integrity constraints in Oracle and is used to ensure that a column cannot have a NULL value. By enforcing this constraint, you guarantee that a column must always contain a valid, non-null value whenever a record is inserted or updated.

Key Points to Understand about the NOT NULL Constraint

1.     Purpose:

• The NOT NULL constraint is used to enforce that a column must always have a value. This is important to ensure that no NULL values are inserted into a column, which could lead to data integrity issues.
• The column will not accept NULL values, meaning data must always be provided for that column.

2.     Syntax: When creating a table, you define the NOT NULL constraint on individual columns using the following syntax:

CREATE TABLE table_name (

    column_name datatype NOT NULL,

    ...

);

Alternatively, you can add a NOT NULL constraint to an existing table:

ALTER TABLE table_name

MODIFY column_name datatype NOT NULL;

3.     When to Use:

• You would use the NOT NULL constraint when you want to ensure that a column always contains valid data (i.e., a value that is not missing).
• It is particularly useful for columns like primary keys or mandatory fields (such as email addresses or phone numbers).

4.     Behavior of NOT NULL Constraint:

o   Insertions: When you insert data into a table with a NOT NULL column, you must provide a value for that column. If you attempt to insert a NULL value into a NOT NULL column, Oracle will raise an error. Example:

INSERT INTO employees (employee_id, first_name, last_name)

VALUES (1, 'John', NULL);

If the last_name column is defined with a NOT NULL constraint, Oracle will raise an error when attempting to insert a NULL value.

o   Updates: When you update a table and modify a column defined with NOT NULL, the updated value must also be non-NULL. Example:

UPDATE employees

SET last_name = NULL

WHERE employee_id = 1;

This query will fail if the last_name column has a NOT NULL constraint.

5.     Defining the NOT NULL Constraint in Table Creation:

The NOT NULL constraint can be directly defined in the CREATE TABLE statement:

CREATE TABLE employees (

    employee_id NUMBER PRIMARY KEY,

    first_name VARCHAR2(50) NOT NULL,

    last_name VARCHAR2(50) NOT NULL,

    hire_date DATE

);

6.     Combining NOT NULL with Other Constraints:

• Primary Key: A PRIMARY KEY constraint automatically implies the NOT NULL constraint. So, when you define a column as a primary key, Oracle will ensure that this column is never null.
• Foreign Key: A FOREIGN KEY constraint does not automatically imply a NOT NULL constraint, but often columns with foreign keys are required to be NOT NULL to ensure relational integrity.
• Unique: A column with a UNIQUE constraint can still allow NULLs unless explicitly stated as NOT NULL. In some cases, NULLs might be allowed in UNIQUE columns.

7.     Impact on Indexing:

• Columns with a NOT NULL constraint can be indexed just like any other column. This can improve the performance of queries that filter based on this column.
• Nullability and Indexes: Indexes created on columns with a NOT NULL constraint are more efficient when filtering out NULLs since the database doesn't have to handle NULL values.

8.     Handling NOT NULL with Default Values:

• If you define a column as NOT NULL, you can also provide a default value. This ensures that if no value is provided during insertion, the default value will be used. Example:

CREATE TABLE employees (

    employee_id NUMBER PRIMARY KEY,

    first_name VARCHAR2(50),

    last_name VARCHAR2(50) DEFAULT 'Doe' NOT NULL,

    hire_date DATE

);

9.     Handling NULLs and Data Integrity:

• A NOT NULL constraint is a fundamental mechanism to ensure that certain columns always have meaningful data, preserving data integrity by eliminating the possibility of NULL values that might be ambiguous or create inconsistencies.
• By restricting NULL values, you ensure that all rows of your table are complete and that no fields are unintentionally left empty.

10.  Exceptions:

• Some columns might need to allow NULL values for legitimate reasons, such as optional data or incomplete information, so using the NOT NULL constraint should be done with caution.

Advantages of NOT NULL Constraint:

• Ensures data integrity by preventing NULL values in critical columns.
• Ensures that mandatory fields are always filled, such as user identification numbers or product codes.
• Helps improve query performance by eliminating the need to check for NULLs during query execution.

Disadvantages or Considerations:

• Adding a NOT NULL constraint to an existing column with NULL values will fail unless all existing NULL values are updated to non-null values.
• It may introduce flexibility issues if the application needs to handle optional data and cannot provide default values.