SQL NOT NULL Constraint
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:
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
ALTER TABLE Persons
ALTER Age int NOT NULL;
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