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