NOT NULL Keyword

The NOT NULL keyword or constraint is used to ensure that no null values are allowed in a column.

If NOT NULL constraints are applied to that column, you cannot insert or update it without adding value.

SQL NOT NULL ON CREATE TABLE

SQL
CREATE TABLE <TABLE_NAME>(
    <COLUMN_NAME> <DATATYPE> NOT NULL
);

Example

SQL
CREATE TABLE Employee(
    Emp_Id Integer Primary Key,
    Emp_Name Varchar(50) NOT NULL,
    Emp_Salary Integer NOT NULL,
);

SQL NOT NULL ON ALTER TABLE

SQL
ALTER TABLE <TABLE_NAME>
MODIFY <COLUMN_NAME> <DATATYPE> NOT NULL;

Example

SQL
ALTER TABLE Employee
MODIFY Emp_Dept Varchar(50) NOT NULL;

Summary

The NOT NULL keyword or constraint is used to prevent users from adding null values to the column.