Sql-server – Is Unique key Clustered or Non-Clustered Index in SQL Server

clustered-indexsql serverunique-key

I am new to SQL Server and while learning about clustered index, I got confused!

Is unique key clustered or a non-clustered index? Unique key holds only unique values in the column including null, so according to this concept, unique key should be a clustered index, right? But when I went through this article I got confused MSDN

When you create a UNIQUE constraint, a unique nonclustered index is
created to enforce a UNIQUE constraint by default. You can specify a
unique clustered index if a clustered index on the table does not
already exist.

Please help me to understand the concept in a better manner, Thank you.

Best Answer

There are three ways of enforcing uniqueness in SQL Server indexes.

Primary Key constraint
Unique constraint
Unique index (not constraint based)

Whether they are clustered or non clustered is orthogonal to whether or not the indexes are declared as unique using any of these methods.

All three methods can create a clustered or non clustered index.

By default the unique constraint and Unique index will create a non clustered index if you don't specify any different (and the PK will by default be created as CLUSTERED if no conflicting clustered index exists) but you can explicitly specify CLUSTERED/NONCLUSTERED for any of them.

Example syntax is

CREATE TABLE T
(
X INT NOT NULL,
Y INT NOT NULL,
Z INT NOT NULL
);

ALTER TABLE T ADD PRIMARY KEY NONCLUSTERED(X);

--Unique constraint NONCLUSTERED would be the default anyway
ALTER TABLE T ADD UNIQUE NONCLUSTERED(Y); 

CREATE UNIQUE CLUSTERED INDEX ix ON T(Z);

DROP TABLE T;

For indexes that are not specified as unique SQL Server will silently make them unique any way. For clustered indexes this is done by appending a uniquefier to duplicate keys. For non clustered indexes the row identifier (logical or physical) is added to the key to guarantee uniqueness.

Syntax:

ALTER TABLE {TABLENAME} 
ADD {COLUMNNAME} {TYPE} {NULL|NOT NULL} 
CONSTRAINT {CONSTRAINT_NAME} DEFAULT {DEFAULT_VALUE}
WITH VALUES

Example:

ALTER TABLE SomeTable
        ADD SomeCol Bit NULL --Or NOT NULL.
 CONSTRAINT D_SomeTable_SomeCol --When Omitted a Default-Constraint Name is autogenerated.
    DEFAULT (0)--Optional Default-Constraint.
WITH VALUES --Add if Column is Nullable and you want the Default Value for Existing Records.

Notes:

Optional Constraint Name:
If you leave out CONSTRAINT D_SomeTable_SomeCol then SQL Server will autogenerate
a Default-Contraint with a funny Name like: DF__SomeTa__SomeC__4FB7FEF6

Optional With-Values Statement:
The WITH VALUES is only needed when your Column is Nullable
and you want the Default Value used for Existing Records.
If your Column is NOT NULL, then it will automatically use the Default Value
for all Existing Records, whether you specify WITH VALUES or not.

How Inserts work with a Default-Constraint:
If you insert a Record into SomeTable and do not Specify SomeCol's value, then it will Default to 0.
If you insert a Record and Specify SomeCol's value as NULL (and your column allows nulls),
then the Default-Constraint will not be used and NULL will be inserted as the Value.

Notes were based on everyone's great feedback below.
Special Thanks to:
@Yatrix, @WalterStabosz, @YahooSerious, and @StackMan for their Comments.

Sql – How to return only the Date from a SQL Server DateTime datatype

SELECT DATEADD(dd, 0, DATEDIFF(dd, 0, @your_date))

for example

SELECT DATEADD(dd, 0, DATEDIFF(dd, 0, GETDATE()))

gives me

2008-09-22 00:00:00.000

Pros:

No varchar<->datetime conversions required
No need to think about locale

Best Answer

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Sql – Add a column with a default value to an existing table in SQL Server

Syntax:

Example:

Notes:

Sql – How to return only the Date from a SQL Server DateTime datatype

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