SQL NOT NULL Constraint

By default, a column can hold NULL values.

The NOT NULL constraint enforces a column to NOT accept NULL values.

This enforces a field to always contain a value, which means that you cannot insert a new record, or update a record without adding a value to this field.

SQL NOT NULL on CREATE TABLE

The following SQL ensures that the "ID", "LastName", and "FirstName" columns will NOT accept NULL values when the "Persons" table is created:

Example

CREATE TABLE Persons (
    ID int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255) NOT NULL,
    Age int
);

SQL NOT NULL on ALTER TABLE

To create a NOT NULL constraint on the "Age" column when the "Persons" table is already created, use the following SQL:

SQL Server / MS Access:

ALTER TABLE Persons
ALTER COLUMN Age int NOT NULL;

My SQL / Oracle (prior version 10G):

ALTER TABLE Persons
MODIFY COLUMN Age int NOT NULL;

Oracle 10G and later:

ALTER TABLE Persons
MODIFY Age int NOT NULL;

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