Relational keys
- 1. Name: Sana Salah_ud_din M.Sc CS (final) Roll#: 138005 Topic: Keys In Relational Database
- 2. AT A Glance: Ø What is : Ø A Key ? Ø A Super Key ? Ø A Candidate Key ? Ø A Primary Key ? Ø An Alternate Key ? Ø
- 3. relational Key: A KEY is an attribute or set of attributes that uniquely identifies a TUPLE in a relation. There are two main types of key: A Super Key and a Foreign key Relational Keys Super Key Foreign Key Candidate Key Primary Key Alternate key Candidate Key
- 4. Super Key: An attribute, or set of attributes, that uniquely identifies a tuple within a relation. A Super key may contain additional attributes that are not necessary for unique identification. A super key is the most general type of key. EXAMPLE: As shown in the figure the relat- ion Branch has a super key
- 5. Candidate key: A key which is the candidate for being selected as a primary key. A candidate key contains only the attributes which can uniquely identify a tuple in a relation. As shown in the figure --- in the relation Branch the attributes branchNo & postcode can serve as candidate keys because both can uniquely identify a tuple in
- 6. Primary key: The candidate key that is selected to identify tuples uniquely within the relation. Some most important points about primary key are: A relation can have only one primary key. Each value in primary key attribute must be unique. Primary key cannot contain null values. As shown in the figure---- the attribute branchNo can serve as primary key because
- 7. Alternate key: The candidate keys that are not selected as primary key are known as alternate keys. As shown in the figure the relation Branch has branchNo serving as pri- mary key and postcode serving as alternate key because postcode can
- 8. Composite Key: A primary key that consists of two or more attributes is known as COMPOSITE KEY. As shown in the figu- re– the relation Branch has two attributes serving as primary key so this is a composite key,
- 9. Foreign key: An attribute, or set of attributes, within one relation that matches the candidate key of some relation.. As shown in the figur- e in the relation Staff The attribute branchNo is the foreign key because it is the prim- ary key in the relation Branch.
- 10. The end
Editor's Notes
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