sql server - Unique Constraint in SQL with Multiple NULL Values -

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i read way ensure unique values in column in sql while allowing multiple nulls.

done using filtered indexes:

create unique index indexname on tablename(columns) include includecolumns  columnname not null

could explains how works?

unique constraint created on column or not ?

to answer first question: when index filtered, doesn't fix criteria in clause left out of index.
if index unique, uniqueness enforced on data fits criteria in clause.

to answer second question: in sql server unique constraints implemented creating unique indexes under hood, there not difference between them. in case uniqueness enforced on index , not directly on table column.


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