Keys in RDBMS (Solved Questions)

Summary

This video provides a comprehensive overview of keys in Relational Database Management Systems (RDBMS). It covers key concepts such as primary, foreign, candidate, and superkeys, explaining their roles in ensuring data integrity and establishing relationships between tables. Through multiple-choice questions, the video explores real-world examples of keys, including how to choose appropriate keys for different tables, and how composite keys work. It also delves into conditions for defining candidate keys and the limitations on defining multiple primary keys in a table. A great resource for understanding key principles in database management.

Takeaways

• 😀 Keys in RDBMS are essential for uniquely identifying records in tables, and can be a combination of one or more columns.
• 😀 A key is not mandatory in every table, as some tables may not require a key attribute.
• 😀 Keys in a table should not be changed frequently, though in rare cases, they might be adjusted.
• 😀 The Employee ID is an appropriate key attribute for identifying employees in a table, as it is unique across the organization.
• 😀 Composite keys can be formed by combining multiple attributes (e.g., name + date of birth), but they may not always guarantee uniqueness.
• 😀 A **candidate key** is a minimal super key, meaning it contains only the essential attributes needed to uniquely identify a record.
• 😀 A **super key** is a set of attributes that can uniquely identify a record, but may include redundant data.
• 😀 The primary key is selected from the candidate keys, and it ensures data uniqueness. A table can have only one primary key.
• 😀 Foreign keys are used to create relationships between two tables by referencing the primary key of another table.
• 😀 It is not possible to define more than one primary key in a table, but a table can have multiple candidate keys, from which only one will be chosen as the primary key.
• 😀 A proper subset of a super key cannot be considered a super key unless it is minimal, making it a candidate key.

Q & A

What is the main purpose of keys in RDBMS?

The main purpose of keys in RDBMS is to uniquely identify records within a table and establish relationships between tables.

What makes Option C the correct answer in Question 1 (Which of the following is correct about keys in RDBMS)?

Option C is correct because a key can be a set of one or more columns that uniquely identify a record, which is the definition of a key in a relational database.

Why is Option A incorrect in Question 1 (Is it mandatory to have a key in every table)?

Option A is incorrect because not all tables require a key attribute. Some tables might not have a key, depending on the design of the database.

Why is the Employee ID the best choice for a key attribute in Question 2?

The Employee ID is unique for each employee in the organization, ensuring that it can uniquely identify each record in the table.

In Question 3, why is the correct answer Option D: 'None of the above' for the student table schema?

Option D is correct because none of the combinations (name, name + date of birth, or name + city) can guarantee uniqueness. Multiple students could have the same name or city, and date of birth may not be unique enough to serve as a key.

What is a candidate key, as described in Question 4?

A candidate key is a minimal set of attributes that can uniquely identify a tuple (record) in a table. It does not have any unnecessary or extraneous attributes.

What does it mean for a super key to be minimal, as explained in Question 4?

A super key is a set of attributes that can uniquely identify a record, but it may contain extra attributes. A candidate key is a minimal super key, meaning it contains no unnecessary attributes.

In Question 5, why is Option C ('No proper subset is a super key') the correct answer?

Option C is correct because a candidate key is a minimal super key, meaning that none of its proper subsets can function as a super key. It must contain only the essential attributes needed for uniqueness.

Why can't we define more than one primary key in a table, as discussed in Question 6?

A table can only have one primary key because the primary key is the single unique identifier for a record in the table. However, a table can have multiple candidate keys, but only one of them will be chosen as the primary key.

What role does a foreign key play in relational databases, as explained in Question 7?

A foreign key is used to establish and enforce relationships between two tables. It refers to the primary key in another table, ensuring referential integrity.