2. SQL Data Types : Nulls
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A null represents an undefined or unknown value and is specified by the keyword null. A null is not the same as a zero, a blank, or an empty string.
A null can be assigned to any nullable column when no other value is specifically assigned. More information about defining nullable columns is provided in the section Create Table in the chapter “SQL Statements.”
The IFNULL function and the IS NULL Predicate allow nulls in queries to be handled.