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PRIMARY KEY |
UNIQUE KEY |
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Basic Difference |
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1 |
Primary key cannot have a NULL value. Because of this we refer:
PRIMARY KEY = UNIQUE KEY + Not Null CONSTRAINT.
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Unique Constraint may have only one NULL value. |
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2 |
Each table can have only one primary key. |
Each table can have more than one Unique Constraint. |
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3 |
By default it adds a clustered index. But as per required we can create with none clustered index. |
By default it adds a UNIQUE non-clustered index. But as per required we can create with clustered index. |
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Advance Difference |
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4 |
Primary key can be related with another table's as a Foreign Key. |
Unique Constraint can not be related with another table's as a Foreign Key. |
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5 |
We can generated ID automatically with the help of Auto Increment field. Primary key supports Auto Increment value.
A primary key must be unique.
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Unique Constraint doesn't supports Auto Increment value. |
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6 |
A primary key must be unique. |
A unique key does not have to be the primary key |
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7 |
We can't delete primary key value from the parent table which is used as a foreign key in child table. To delete we first need to
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Not applied |
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