A NULL value means no value on the data row. By default column of the table hold a NULL value.
NOT NULL Constraint enforce the column not accept a NULL value. It is a way to add another layer of validation of data.
MySQL NOT NULL Constraint avoids NULL value but It can accept the duplicate value.
The syntax for NOT NULL:-
column_name datatype NOT NULL
Example:-
CREATE TABLE Employee(
ID INT AUTO_INCREMENT PRIMARY KEY,
Name varchar(500) NOT NULL,
Department varchar(255),
);
NOT NULL for ALTER table:-
ALTER TABLE Employee MODIFY Department varchar(255) NOT NULL;