how to write not null in sql code example

Example 1: sql not null

SELECT column_name FROM table_name 
WHERE column_name IS NOT NULL;

Example 2: how to check if a column is null in sql

SELECT column_name FROM table_name 
WHERE column_name IS NULL;

Example 3: NOT NULL SQL

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 in creating a table
CREATE TABLE Persons (
    ID int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255) NOT NULL,
    Age int
);