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"Unique Key allows only one NULL Value".
somebody can give some example or justify the statement how its working.

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Just think again what you want to achieve.
A (unique) key is used for identifying a record. Any value of the key must exist only one time - how else could it identify a record? When two records have a key value of null, which of the two records is the record identified by null? => better gid rid of the idea of a nullable key, it's confusing only.
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