Unique Index and Unique Constraint are the same. They achieve same goal. SQL Performance is same for both.
Add Unique Constraint
ALTER TABLE dbo.<tablename> ADD CONSTRAINT
<namingconventionconstraint> UNIQUE NONCLUSTERED
(
<columnname>
) ON [PRIMARY]
Add Unique Index
CREATE UNIQUE NONCLUSTERED INDEX
<namingconventionconstraint> ON dbo.<tablename>
(
<columnname>
) ON [PRIMARY]
There is no difference between Unique Index and Unique Constraint. Even though syntax are different the effect is the same. Unique Constraint creates Unique Index to maintain the constraint to prevent duplicate keys. Unique Index or Primary Key Index are physical structure that maintain uniqueness over some combination of columns across all rows of a table. It is a convenient way to enforce a Unique Constraint for SQL Server.
Reference : Pinal Dave (http://blog.SQLAuthority.com)
pinaldave
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From MSDN: “The index creation options other than FILLFACTOR that are available for a unique index are not available for a unique constraint.”
So yes, their effects are the same, but there are differences.
Thank you Alex,
You are correct.
Very informative.
I am new in thsi field, so may i ask.
1. how could i make a query with the index.
2. Is there ever a way where i can use this command in visual basic?
thanks and more power
“The index creation options other than FILLFACTOR that are available for a unique index are not available for a unique constraint.”
Not true for 2005. Try this –
create table Emp (Id int, Name varchar(20))
GO
ALTER TABLE Emp
ADD CONSTRAINT IX_Emp
UNIQUE (Name)
WITH (IGNORE_DUP_KEY = ON)
GO
insert into Emp (Id, Name)
select 1, ‘Me’
union
select 2, ‘Me’
GO
select * from Emp
But I agree that no all options for an index are available for a constraint, such as DROP_EXISTING.
You can use a column with Unique Constraint to be a foreign key for another table but you cant do that with Unique Index
Another difference. A unique index can have nullable columns as part of the index. No column in a primary key can nullable.
Creating a unique constraint implicitly creates a unique index on the table with the same name. Creating a unique index does not implicitly create a unique constraint.
Like all constraints, Unique constraints have to be uniquely named within the database.
Like all indexes, Unique indexes only have to be uniquely named within the table they owned by.
Since creating the unique constraint also creates a unique index on the table, you cannot use the same name for a unique constraint and a unique index (or any constraint and any index for that matter) on the same table.
Also, GPH is correct that -primary key- constraints can’t have nullable columns. However, Primary key constraints are not the same as unique constraints. Unique constraints can have nullable columns like unique indexes.
hello,
Plz tell me what is the difference between unique key and primary key
also tell me how could i download the small date bank and sql at home because i am learning it in germany and also working in a company plz tell me how could i free download .
Hello,
FYI,
In fact, unique key allows single nulls.
E.g.
ID-Name(AS UNIQUE KEY)
1-Null
2-Joyal
3-Abhay
Thanks.
Regards,
Joyal
want to know, while inserting multiple records in a table under one transaction at what time a unique constraint and a unique index will check for uniqueness. As Books online suggests to create Unique constraint instead of Unique index, it gave me some idea, perhaps unique key checks uniqueness after every insert while unique index verifies after inserting all records. But i still want to know the functionality of the both.
How can i add a unique cnstraint to table on
combination of bit and varchar datatypes
Thanks in advance
Table_A:
seqid (Primary Key)
cmpno, prdno (Unique key)
TABLE_B:
bcompno, bprdno (Refers cmpno, prdno of TABLE_A)
Can I have a Foreign Key relationship between TABLE_B and TABLE_A, using Unique Key, in SQL Server 2005?
Can we create a uiique constraint without an uniqueindex when i am trying to drop a unique index which is created along with unique constraint it is giving error
Surprisingly when I created unique index on one my tables it did not stop me to insert duplicates, while the unique constraint did the trick. Can you please explain why it happened?
kind regards,
Azim
Hi Amar Kumar ,
The main difference betn P.K and unique key is P.K. does not allow null value,where unique key allows null values.
When P.K is created at tht time cluster index is created autmatically and when unique key is created at that time non clustr index is created.
Hi Pinal ,
the one more difference betn unique index and unique constraint is index is used to fetch the data fast. tht is use for improve the performance of fetching the data. where unique constraint is used to prevent the entering the redudant data in to the table.
Hi Symphani,
No u can’t use like that . bcz F.K is only reference to the P.K. Yes P.K is unique key but unique is not P.K.
Hi Pardeepe,
U can define unique key on varchar datatype but u can’t do it on bit datatype bcz bit datatype has values 0,1,null .
please explain how the value ‘a’ and ‘a ‘ are treated the same in SQL Server 2005 when creating indexes:
create table a(b nvarchar(5))
insert into a values (‘a’)
insert into a values (‘a ‘) — please note space at the end
create unique index a1 on a(b)
gives error:
Msg 1505, Level 16, State 1, Line 2
The CREATE UNIQUE INDEX statement terminated because a duplicate key was found for the object name ‘dbo.a’ and the index name ‘a1′. The duplicate key value is (a ).
Darshan,
I’m afraid that is incorrect. A foreign key may reference the primary key of the table *or* any column(s) defined as unique.
Tushar,
In Sql Server, any trailing spaces are ignored when comparing strings. I assume this is so that if a field is defined “char(10)” and another is defined “varchar(10)” and you insert ‘a’ into each one, then CharField = VarCharField will evaluate to TRUE even though the char(10) field actually contains an ‘a’ followed by 9 spaces.
So in statements like “where RTrim(CharField) = RTrim(VarCharField)”, the RTrim is not necessary. Even Len(CharField) and Len(VarCharField) will return 1 for both. However, leading spaces *are* significant.
TommCatt
Thankx for your reply.
Your explanation is correct.
As mentioned by you, I am aware of SQL Server’s handling of various string data types.
In my opinion, ignoring trailing blanks while comparing string objects (aspecially when they are defined as variable length strings is a bad design flow).
I have designed a data warehouse where we are bringing data from Oracle whose table’s have key values e.g. AB and AB. Again having space in a key field itself is a bad design but structurally Oracle treats AB and AB as different values (which infact they are) and handles them OK.
When I bring then over to SQL server, I get unique key violation … vola … :)
My work around is to replace trailing blanks in key fields with ~ character. Again a temporary fix till SQL Server provides a way to have sound string data type comparisons.
Again, Thankx for your reply.
i need to know how work when there is a situation that there is a Unique Key and this field “alow null”, but when i am going to create a Unique Key the SQLSERVER saw that there were values duplicated and the values are “nulls”. How do i sove this problem?
@Cristiano,
As you may be aware, Unique Key can allow only one null. Meaning only one null can be accepted not more than that. If you have more than one null on any column, you cannot make that column/field as unique key.
I am sure there must be another way of doing this, I would do something like this,
1. Since there is no way you can make that field a unique key, how about making a composite key.
a) Combine this field with any other unique column/field and make a composite unique key. If any other unique key already exists then add this field to the previous unique key.
b) if you do not have any unique key ( I am sure you do), but if you dont, then you can do this always, using Enterprise manager or SSMS object Explorer, create a new Identity column in the table and then create a unique composite key on these two field. ( You can create an identity column if you have data in the table, you DONT have to drop and recreate the table, using EM or SSMS you can always create a identity column in the table).
If you consider case a), then make sure there are no duplicates for the combination of new field and the field you chose for unique key.
If you create a composite unique key on cola and colb combined then you can have more than one null in cola and in colb, but again the combination of cola and colb should be unique.
Once you start making a composite unique key… this concept will be more easier.
Try doing this, either in Enterprise Manager or SSMS ( Object Explorer) much easier in interface than executing scripts.
Hope this helps.
Imran.
[...] Cristiano asked following questions : [...]
[...] SQL SERVER – Difference between Unique Index vs Unique Constraint [...]
Just discovered another difference between declaring a unique index vs. a unique constraint.
SQL will raise an error with an error code of 2601 when violating a unique index, while it will use error code 2627 for violating a unique constraint.
Minor difference, but while they are very similar, they are NOT the same.
@Reed
Very interesting!
CREATE TABLE A(A INT UNIQUE);
INSERT INTO A(A) VALUES(1);
INSERT INTO A(A) VALUES(1);
DROP TABLE A;
CREATE TABLE A(A INT);
CREATE UNIQUE INDEX AA ON A(A);
INSERT INTO A(A) VALUES(1);
INSERT INTO A(A) VALUES(1);
DROP TABLE A;