when to use not null in sql code example

Example 1: find value if not null in sql

SELECT FIRST_NAME , MANAGER_ID 
FROM EMPLOYEES 
WHERE MANAGER_ID IS NOT NULL ;

Example 2: sql is not null

The reverse of NULL. Tests for values that aren’t empty / NULL
"Tests for empty (NULL) values.
Example: Returns users that haven’t given a contact number.
SELECT * FROM users
WHERE contact_number 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
);

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