NULL is utilized in table fields to denote missing, unknown, or inapplicable data in a database. When inserting a record, if data for a specific field is not available, it’s better to use NULL instead of inserting a placeholder like 0, an empty string (”), or any random value. Using NULL explicitly indicates that the data is missing or unknown. Not all fields may be required for all rows. Some fields are optional based on business logic, and NULL is a way to mark that no data was entered for that particular field.

The ISNULL function serves different purposes in SQL Server and MySQL. In SQL Server, the ISNULL() function is used to replace NULL values.

--Syntax--
SELECT 
    column(s), 
    ISNULL(column_name, value_to_replace)
FROM table_name;

SELECT user_id, 
    user_name, 
    ISNULL(user_phone, 'xxxxxxxxxx') AS user_phone, 
    ISNULL(Age, 0) AS Age 
FROM users