Oracle NOT NULL

Summary: in this tutorial, you’ll learn how to use the Oracle NOT NULL constraint to ensure a column does not have NULL.

Introduction to Oracle NOT NULL constraint #

In Oracle, NOT NULL constraint specifies that a column cannot contain NULL.

The NOT NULL constraints are inline constraints which are typically used in the column definition of the CREATE TABLE statement:

CREATE TABLE table_name (
    column1 datatype NOT NULL
    ...
);Code language: SQL (Structured Query Language) (sql)

It is possible to add a NOT NULL constraint to an existing table by using the ALTER TABLE statement.

ALTER TABLE table_name MODIFY ( column_name NOT NULL);Code language: SQL (Structured Query Language) (sql)

In this case, the column_name must not contain any NULL value before applying the NOT NULL constraint.

Oracle NOT NULL constraint examples #

First, create a new table called the surcharges:

CREATE TABLE surcharges (
  surcharge_id NUMBER GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
  surcharge_name VARCHAR2(255) NOT NULL,
  amount NUMBER(9,2)
);Code language: SQL (Structured Query Language) (sql)

The surcharges table has three columns: surcharge id, surcharge name, and amount.

• The surcharge_id column is the primary key column of the table specified by the PRIMARY KEY constraint, therefore, Oracle implicitly adds a NOT NULL constraint to this column.
• The surcharge_name column has a NOT NULL constraint specified explicitly in the column definition.
• The amount column can accept NULL values.

Second, insert a row into the surcharges table:

INSERT INTO surcharges(surcharge_name, amount)
VALUES('Late order placement',10);Code language: SQL (Structured Query Language) (sql)

It works as expected.

However, the following statement does not work:

INSERT INTO surcharges(surcharge_name, amount)
VALUES(null,20);Code language: SQL (Structured Query Language) (sql)

The reason is that it attempts to insert NULL into the surcharge column which has a NOT NULL constraint.

The following statement works because the amount column accepts NULL:

INSERT INTO surcharges(surcharge_name, amount)
VALUES('Rush Order',NULL);Code language: SQL (Structured Query Language) (sql)

The following statement displays all constraints of the surcharges table:

SELECT
    table_name,
    constraint_name,
    search_condition
FROM
    user_constraints
WHERE
    table_name = 'SURCHARGES'; 
Code language: SQL (Structured Query Language) (sql)

If you want to add a NOT NULL constraint to the amount column, you use the following ALTER TABLE statement:

ALTER TABLE surcharges MODIFY (amount NOT NULL);Code language: SQL (Structured Query Language) (sql)

The following error occurred because the surcharges table contains NULL:

SQL Error: ORA-02296: cannot enable (OT.) - null values foundCode language: SQL (Structured Query Language) (sql)

So before adding the NOT NULL constraint, you need to make sure that the existing data in the surcharges table does not violate the NOT NULL constraint:

UPDATE
    surcharges
SET
    amount = 0
WHERE
    amount IS NULL;Code language: SQL (Structured Query Language) (sql)

Now, if you execute the ALTER TABLE statement again:

ALTER TABLE surcharges MODIFY (amount NOT NULL);Code language: SQL (Structured Query Language) (sql)

It should work as expected.

Drop NOT NULL constraints #

Sometimes, you need to change a column with a NOT NULL constraint to accept NULL values.

To do this, you need to remove the NOT NULL constraint from the column by using the ALTER TABLE statement as below:

ALTER TABLE table_name MODIFY ( column_name NULL)Code language: SQL (Structured Query Language) (sql)

For example, to drop the NOT NULL constraint from the amount column of the surcharges table, you use the following statement:

ALTER TABLE surcharges
MODIFY (amount NULL);Code language: SQL (Structured Query Language) (sql)

Summary #

• Use the Oracle NOT NULL constraint to enforce a column not to accept NULL values.