unique key related

One of the major purposes of a UNIQUE constraint is to prevent duplicate records. Instead of "Null value", if we use phrase "null marker" - a marker in a column indicating that the actual value in this column is either missing or inapplicable. Logically, any key which is allowed to contain non duplicate (unique) values is a unique key, NULL is a permissible value in sql server , so it can have NULL for a single time just like any other value.

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