Check out example codes for "NOT NULL SQL". It will help you in understanding the concepts better.

Code Example 1

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

Code Example 2

SELECT column_names
FROM table_name
WHERE column_name IS NOT NULL;

Code Example 3

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
);

Code Example 4

SELECT column_name FROM table_name 
WHERE column_name IS NULL;

Code Example 5

ALTER TABLE Persons
MODIFY Age int NOT NULL;

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