SQL NOT NULL

SQL NOT NULL Constraint

By default, columns can contain NULL values.

The NOT NULL constraint ensures a column cannot accept them, mandating that every field must have a value.

Thus, inserting or updating records requires specifying a value for this field.

The subsequent SQL guarantees that the “ID”, “LastName”, and “FirstName” columns will not allow NULL values upon creating the “Persons” table:

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

To apply a NOT NULL constraint to the “Age” column after the creation of the “Persons” table, utilize 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;