They both uniquely identify the tuple in table in Database.
But there is some difference between them as
1. The not null constraint is by default added to primary key, it means, primary key attribute cannot accept null values, Primary Key can’t accept null values but unique key can accept only one null value.
2. We can have only one Primary key in a table but we can have more than one unique key in a table.
3. By default, Primary key is clustered index but By default, Unique key is a unique non-clustered index.
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