Keys and its Importance in DBMS and RDBMS
- 1. Module I – Session 3 Keys and It’s Importance Presented by Dr.S.Zulaikha Beevi, Professor/CSE.
- 2. RDBMS
- 4. Keys and its importance Keys play an important role in the relational database. It is used to uniquely identify any record or row of data from the table. It is also used to establish and identify relationships between tables. For example: In Student table, ID is used as a key because it is unique for each student. In PERSON table, passport_number, license_number, SSN are keys since they are unique for each person.
- 6. Types of Key
- 7. Super Key Super Key is the superset of primary key. The super key contains a set of attributes, including the primary key, which can uniquely identify any data row in the table. For example: In the above EMPLOYEE table, for(EMPLOEE_ID, EMPLOYEE_NAME) the name of two employees can be the same, but their EMPLYEE_ID can't be the same. Hence, this combination can also be a key. The super key would be EMPLOYEE-ID, (EMPLOYEE_ID, EMPLOYEE-NAME), etc.
- 8. Candidate key Candidate Key - The candidate keys in a table are defined as the set of keys that is minimal and can uniquely identify any data row in the table. For example: In the EMPLOYEE table, id is best suited for the primary key. Rest of the attributes like SSN, Passport_Number, and License_Number, etc. are considered as a candidate key.
- 9. Primary Key The primary key is selected from one of the candidate keys and becomes the identifying key of a table. It can uniquely identify any data row of the table. In the EMPLOYEE table, ID can be primary key since it is unique for each employee. In the EMPLOYEE table, we can even select License_Number and Passport_Number as primary key since they are also unique.
- 10. Foreign key A foreign key is an attribute value in a table that acts as the primary key in another table. Hence, the foreign key is useful in linking together two tables. In a company, every employee works in a specific department, and employee and department are two different entities. So we can't store the information of the department in the employee table. That's why we link these two tables through the primary key of one table. We add the primary key of the DEPARTMENT table, Department_Id as a new attribute in the EMPLOYEE table. Now in the EMPLOYEE table, Department_Id is the foreign key, and both the tables are related.
- 12. Composite Key - If any single attribute of a table is not capable of being the key i.e it cannot identify a row uniquely, then we combine two or more attributes to form a key. This is known as a composite key. Secondary Key - Only one of the candidate keys is selected as the primary key. The rest of them are known as secondary keys.
- 14. Qui z 1. . A in a table represents a relationship among a set of values. a) Column b) Key c) Row d) Entry Answer: C
- 15. 2. The term attribute refers to a of a table. a) Record b) Column c) Tuple d) Key Answer: B