Unique index or unique key?

Question

What is the diffrence between a unique index and a unique key?

Answer 1

The unique piece is not where the difference lies. The index and key are not the same thing, and are not comparable.

A key is a data column, or several columns, that are forced to be unique with a constraint, either primary key or explicitly defined unique constraint. Whereas an index is a structure for storing data location for faster retrieval.

From the docs:

Unique Index

Creates a unique index on a table or view. A unique index is one in which no two rows are permitted to have the same index key value. A clustered index on a view must be unique

Unique key (Constraint)

You can use UNIQUE constraints to make sure that no duplicate values are entered in specific columns that do not participate in a primary key. Although both a UNIQUE constraint and a PRIMARY KEY constraint enforce uniqueness, use a UNIQUE constraint instead of a PRIMARY KEY constraint when you want to enforce the uniqueness of a column, or combination of columns, that is not the primary key.

Answer 2

This MSDN article comparing the two is what you're after. The terminology is such that "constraint" is ANSI, but in SQL Server you can't disable a Unique Constraint...

For most purposes, there's no difference - the constraint is implemented as an index under the covers. The MSDN article backs this up--the difference is in the meta-data, for things like:

• tweaking FILLFACTOR
• INCLUDE provides more efficient covering indexes (composite constraint)
• A filtered index is like a constraint over a subset of rows/ignore multiple null etc.