SQL
NULL Values

Null values is field that has no value. If a field in a table is optional, we can insert a new record or update record without adding a value to this field.

IS NULL

Syntax
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    SELECT column_names
	FROM table_name
	WHERE column_name IS NULL;
Example
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    SELECT CustomerName, ContactNo, Address
	FROM Customers
	WHERE ContactNo IS NULL;

NOT NULL

Syntax
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    SELECT column_names
	FROM table_name
	WHERE column_name IS NOT NULL;
Example
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    SELECT CustomerName, ContactNo, Address
	FROM Customers
	WHERE ContactNo IS NOT NULL;