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When I want a column to have distinct values, I can either use a constraint

create table t1(
id int primary key,
code varchar(10) unique NULL
);
go

or I can use a unique index

create table t2(
id int primary key,
code varchar(10) NULL
);
go

create unique index I_t2 on t2(code);

Columns with unique constraints seem to be good candidates for unique indexes.

Are there any known reasons to use unique constraints and not to use unique indexes instead?

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4  
are they actually different? I think in some databases e.g. postgresql, a unique constraint simply creates a unique index. I'm not answering because I know nothing about sql server. –  xenoterracide Jan 4 '11 at 11:10
1  
in postgresql, you can use an expression in a unique index but not in a unique constraint. –  Neil McGuigan Dec 16 '13 at 22:35

5 Answers 5

up vote 41 down vote accepted

Under the hood a unique constraint is implemented the same way as a unique index - an index is needed to efficiently fulfill the requirement to enforce the constraint. Even if the index is created as a result of a UNIQUE constraint, the query planner can use it like any other index if it sees it as the best way to approach a given query.

So for a database that supports both features the choice of which to use will often come down to preferred style and consistency.

If you are planning to use the index as an index (i.e. your code may rely on searching/sorting/filtering on that field to be quick) I would explicitly use a unique index (and comment the source) rather than a constraint to make that clear - this way if the uniqueness requirement is changed in a later revision of the application you (or some other coder) will know to make sure a non-unique index is put in place of the unique one (just removing a unique constraint would remove the index completely). Also a specific index can be named in an index hint (i.e. WITH(INDEX(ix_index_name)), which I don't think is the case for the index created behind the scenes for managing uniqueness as you are unlikely to know its name.

Likewise if you are only needing to enforce uniqueness as a business rule rather than the field needing to be searched or used for sorting then I'd use the constraint, again to make the intended use more obvious when someone else looks at your table definition.

Note that if you use both a unique constraint and a unique index on the same field the database will not be bright enough to see the duplication, so you will end up with two indexes which will consume extra space and slow down row inserts/updates.

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I'm wondering about the "the database will not be bright enough"? Is that true for all RDBMS? Is it mandated by the SQL-Standard? And even if it is (and I'd wonder why it should be), do all implementations implement it that way? Or: why can't a DB be "bright" enough? –  Jürgen A. Erhard Jan 4 '11 at 12:20
4  
@jae: a DBMS certainly could be bright enough, but you'd have to check with each DBMS to see if it is. If you ask MSSQL to create two identical indexes it will create two rather than one referred to by two names (at least this was the case last time I spotted a situation like that (due to a copy+paste error on my part)), so I assume the same is the case if one of the indexes is present due to a constraint. –  David Spillett Jan 4 '11 at 12:51
    
+1 @David Spillett I think that basically the DBMS just assumes you know what you are doing; If you feel like creating the same index twice, it doesn't question you on that. –  Andrew Barber Jan 11 '11 at 22:32
1  
@TFRsFM: I can't speak of Derby specifically but I'm pretty sure mySQL operates this way. Probably all relational database systems do. –  David Spillett May 18 '11 at 23:10
1  
You can name a constraint and use it in an index hint. CREATE TABLE #T(X INT CONSTRAINT PK PRIMARY KEY NONCLUSTERED);SELECT * FROM #T WITH(INDEX(PK)) WHERE X = 1. Indexes can be more flexible though in that constraints don't support all index options such as INCLUDEd columns or filtered indexes. –  Martin Smith Feb 8 '13 at 11:22

In addition to the differences posted in other answers (including some that I think were unfairly downvoted) there are some key differences between the two.

Note: The error messages are from SQL Server 2012.

Errors

Violation of a unique constraint returns error 2627.

Msg 2627, Level 14, State 1, Line 1
Violation of UNIQUE KEY constraint 'P1U_pk'. Cannot insert duplicate key in object 'dbo.P1U'. The duplicate key value is (1).
The statement has been terminated.

Violation of a unique index returns error 2601.

Msg 2601, Level 14, State 1, Line 1
Cannot insert duplicate key row in object 'dbo.P1' with unique index 'P1_u'. The duplicate key value is (1).
The statement has been terminated.

Disabling

A unique constraint cannot be disabled.

Msg 11415, Level 16, State 1, Line 1
Object 'P1U_pk' cannot be disabled or enabled. This action applies only to foreign key and check constraints.
Msg 4916, Level 16, State 0, Line 1
Could not enable or disable the constraint. See previous errors.

A unique index can be disabled.

ALTER INDEX P1_u ON dbo.P1 DISABLE ;

Options

Unique constraints support indexing options like FILLFACTOR and IGNORE_DUP_KEY, though this hasn't been the case for every versions of SQL Server.

Filtering

A Unique constraint cannot be filtered.

A unique index can be filtered.

CREATE UNIQUE NONCLUSTERED INDEX Students6_DrivesLicence_u 
ON dbo.Students6( DriversLicenceNo ) WHERE DriversLicenceNo is not null ;

Foreign Key Constraints

A Foreign Key constraint cannot reference a filtered unique index, though it can reference a non-filtered unique index (I think this was added in SQL Server 2005).

Links

http://technet.microsoft.com/en-us/library/aa224827(v=SQL.80).aspx

http://technet.microsoft.com/en-us/library/ms177456.aspx

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In Oracle a major difference is you can create a function-unique index, which is not doable with unique constraints:

For example

create unique index ux_test on my_table (case when amount != 0 then fk_xyz end);

So fk_xyz is only unique for record which have amount != 0.

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1  
In SQL Server (the question's tag), indexes can be filtered with a WHERE clause. CREATE UNIQUE NONCLUSTERED INDEX P4_U ON DBO.P4 ( PID ) WHERE TXT = 'qwert' ; –  Greenstone Walker Dec 16 '13 at 21:23

UNIQUE Constraint is preferred over UNIQUE Index. When the constraint is not unique you need to use a regular or non unique index. Constraint is also another type of index. Index is used for faster access.

Unique Indexes can have where clauses. For example, you can create indexes for every year based on the date column

WHERE Sale_Date BETWEEN '2012-01-01' AND '2012-12-31'

Ravi Ramaswamy

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One of the major differences between a unique constraint and a unique index is that a foreign key constraint on another table can reference columns that make up a unique constraint. This is not true for unique indexes. In addition, unique constraints are defined as part of ANSI standard, while indexes are not. Finally, unique constraint in considered to live in the realm of logical database design (which may be implemented differently by different DB engines) while index is physical aspect. Therefore, unique constraint is more declarative. I'd prefer unique constraint in almost all cases.

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4  
-1 In SQL Server the following is wrong: "a foreign key constraint on another table can reference columns that make up a unique constraint. This is not true for unique indexes". In SQL Server, we can refer FK constraints to unique indexes. –  AlexKuznetsov May 2 '12 at 20:31
1  
The ability for a foreign key constraint to reference a unique index was, I think, added in SQL Server 2005. Many sources, including some pages in BOL, haven't been updated to reflect the changes, so I don't think Dmitry's answer deserves the downvotes. The rest of his answer is spot on - Constraints are ANSI-standard, indexes are not. –  Greenstone Walker Dec 16 '13 at 21:08

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