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Asked: 2026-05-17T08:19:08+00:00

Why does INDEX creation statement have UNIQUE argument? As I understand, the non-clustered index

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Why does INDEX creation statement have UNIQUE argument?

As I understand, the non-clustered index contains a bookmark, a pointer to a row, which should be unique to distinguish even non-unique rows,
so insuring non-clustered index to be unique ?
Correct?

So, do I understand that no-unique index can be only on clustered table? since

  • “A clustered index on a view must be unique” [1]

Since “The bottom, or leaf, level of the clustered index contains the actual data rows of the table” [1], do I understand correctly that the same effect as UNIUE on clustered index can be achieved by unique constraint on (possibly all or part of) columns of a table [2]?

Then, what does bring UNIQUE argument for index?
except confusion to basic concepts definitions [3]

Update:
This is again the same pitfall – explaining something already explained many times based on undefined terms converting all explanation to never-ending guessing game.
Please see my subquestion [4] which is really re-wording of this same question here.

Update2:
The problem is in ambiguous, lacking definitions or improper use of terms in improper contexts. If index is defined as structure serving to (find and) identify/point to real data, then non-unique or NULL indexes do not make any sense. Bye

Cited:
[1]
CREATE INDEX (Transact-SQL)
http://msdn.microsoft.com/en-us/library/ms188783.aspx

[2]
CREATE TABLE (Transact-SQL)
http://msdn.microsoft.com/en-us/library/ms174979.aspx

[3]
Unique index or unique key?
Unique index or unique key?

[4]
what is index and can non-clustered index be non-unique?
what is index and can non-clustered index be non-unique?

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1 Answer

  1. Editorial Team

    The UNIQUE index clause is really just a quirk of syntax in SQL Server and some other DBMSs. In Standard SQL, uniqueness constraints are implemented through the use of the PRIMARY KEY and UNIQUE CONSTRAINT syntax, not through indexes (there are no indexes in standard SQL).

    The mechanism SQL Server uses internally to implement uniqueness constraints is called a unique index. A unique index gets created automatically for you whenever you create a PRIMARY KEY or UNIQUE constraint. For reasons best known to the SQL Server development team they decided to expose the UNIQUE keyword as part of the CREATE INDEX syntax, even though the constraint syntax does the same job.

    In the interests of clarity and standards support I would recommend you avoid creating UNIQUE indexes explicitly wherever possible. Use the PRIMARY KEY or UNQIUE constraint syntax instead.

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