SQL NOT NULLConstraints
By default, table columns accept NULL values.
SQL NOT NULL Constraint
The NOT NULL constraint enforces a column to not accept NULL values.
The NOT NULL constraint forces a field to always contain a value. This means that you cannot insert a new record or update a record without adding a value to this field.
The following SQL ensures that the "ID", "LastName", and "FirstName" columns do not accept NULL values:
Examples
CREATE TABLE Persons (
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255) NOT NULL,
Age int
);
Add NOT NULL Constraint
To add a NOT NULL constraint to the "Age" column in an already created table, use the following:
Examples
ALTER TABLE Persons
MODIFY Age int NOT NULL;
Drop NOT NULL Constraint
To drop a NOT NULL constraint from the "Age" column in an already created table, use the following:
Examples
ALTER TABLE Persons
MODIFY Age int NULL;