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In a relational database, a key is an attribute or a combination of attributes used to uniquely identify a tuple (row) in a relation (table). Keys are fundamental to the design of a relational database because they help maintain data integrity, prevent duplicate records, establish relationships between tables, and support efficient retrieval of data. Different types of keys are used depending on their purpose and properties. The important keys in a relational database include super key, candidate key, primary key, and alternate key. Consider a relation STUDENT with the following attributes: STUDENT(RollNo, Name, Email, Course, Phone) Suppose RollNo and Email are both unique for every student. A possible set of records may be: | RollNo | Name | Email | Course | Phone | | ------ | ---- | --------------------------------------- | ------ | ---------- | | 101 | Ravi | [ravi@gmail.com](mailto:ravi@gmail.com) | BCA | 9876543210 | | 102 | Neha | [neha@gmail.com](mailto:neha@gmail.com) | BCA | 9876543211 | | 103 | Aman | [aman@gmail.com](mailto:aman@gmail.com) | BCA | 9876543212 | A super key is any set of one or more attributes that can uniquely identify each record in a relation. A super key may contain additional attributes that are not necessary for establishing uniqueness. For example, in the STUDENT relation, RollNo is a super key because every student has a different RollNo. Similarly, Email is a super key if every email address is unique. The combination (RollNo, Name) is also a super key because RollNo alone is sufficient to identify a student, even though Name is an unnecessary attribute. Likewise, (RollNo, Email) is a super key. Thus, a relation can have many super keys. A candidate key is a minimal super key. It uniquely identifies every tuple, but none of its attributes can be removed without losing the uniqueness property. In the STUDENT relation, RollNo can be a candidate key because it uniquely identifies each student and contains no unnecessary attribute. Email can also be a candidate key if email addresses are guaranteed to be unique. However, (RollNo, Name) is not a candidate key because Name can be removed and RollNo still uniquely identifies the student. Therefore, candidate keys are selected from the set of __________ ______ __________ _________ _____ _________ _______ _______ _____ ____ ______.
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