A field with a NULL value is a field with no value.
If a field in a table is optional, it is possible to insert a new record or update a record without adding a value to this field.
IS NULL
IS NOT NULL
IS NULL SYNTAX
SELECT column_names
FROM table_name
WHERE column_name IS NULL;
EXAMPLE
SELECT CustomerName, ContactName, Address
FROM Customers
WHERE Address IS NULL;
IS NOT NULL SYNTAX
SELECT column_names
FROM table_name
WHERE column_name IS NOT NULL;
SELECT CustomerName, ContactName, Address
FROM Customers
WHERE Address IS NOT NULL;
SQL Intro and https://www.w3schools.com/sql/sql_quickref.asp