primary and unique CONSTRAINT [Resolved]

Primary Key:
1. Combination of NOT NULL and UNIQUE, So, It will not allow Duplicate data and NULL.
2. It will be CLUSTERED Index by default, If the table doesn't have CLUSTERED Index already on that table. It will be NONCLUSTERED PRIMARY KEY, If the table already have CLUSTERED Index on that table.
3. Only one PRIMARY KEY can be defined on the table.

UNIQUE:
1. It allows only one NULL
2. It will create NONCLUSTERED Index, Even the table doesn''t have CLUSTERED Index already on that table.
3. Multiple UNIQUE can be defined on the table.

Note:
PRIMARY KEY or UNIQUE constraint can be referenced by a FOREIGN KEY constraint.

Cheers
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Cheers
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Senthilns2005, if this helps please login to Mark As Answer. | Alert Moderator

The below video should help you. It explain primary and unique constraints practically.
http://youtu.be/8iIvyNPwruU?hd=1

Senthilns2005, if this helps please login to Mark As Answer. | Alert Moderator

Great Answer PandianS!

Best Regards,

VG
www.TeacherJi.com

Senthilns2005, if this helps please login to Mark As Answer. | Alert Moderator

unique is used to restrict duplicate values but it allows null values so restrict null values we are using not null but it allows duplicate values.
to restrict duplicate values and null values we are using primary key constraint.

Thanks&Regards
LakshmiNarayana Nalluri.

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Primary Key:

Primary Key Restrice Duplicate Values and Null values.Each Table Can have only one primary key.By Default Clustered index is an Primary Key.

Unique Key:

Unique key Restrict Duplicate Values and Allow only one Null values. by Default non Clustered Index is an Unique Key.

Regards
Sriram.R

sriram

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Primary Key:
Prmary Key is the unique identifier of the table. Only Single Primary Key is possible on a table. Primary Key may the single column or combination of the columns. Null values not allowed in Primary Key. By default Clustered Index is created when you define a primary Key on a table.

Unique Key:
Only one NULL values is allowed in Unique key column. It is only on single column.

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