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: 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 MODIFY Age int NOT NULL;

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