Define the following terms: relation schema, relational database schema, domain, relation instance, relation cardinality, and relation degree.
How many distinct tuples are in a relation instance with cardinality 22?
Does the relational model, as seen by an SQL query writer, provide physical and logical data independence? Explain.
What is the difference between a candidate key and the primary key for a given relation? What is a superkey?
The Correct Answer and Explanation is :
Definitions of Key Terms
- Relation Schema:
A relation schema specifies the structure of a relation in a relational database. It defines the relation’s name, the attributes (columns), and the domains (possible values) of those attributes. For example, a relation schema might be represented asStudent(Name, Age, ID). - Relational Database Schema:
A relational database schema is a collection of relation schemas. It defines the overall structure of a relational database, including all relations, attributes, domains, and constraints like primary keys and foreign keys. - Domain:
A domain is the set of permissible values that an attribute can take. For example, the domain of an “Age” attribute might be integers between 0 and 150. - Relation Instance:
A relation instance is a specific set of tuples (rows) in a relation at a given point in time. It represents the actual data stored in a relation. - Relation Cardinality:
The cardinality of a relation is the number of tuples (rows) in a relation instance. - Relation Degree:
The degree of a relation is the number of attributes (columns) in the relation schema.
Number of Distinct Tuples in a Relation Instance with Cardinality 22
The number of distinct tuples in a relation instance with cardinality 22 is 22 because the cardinality represents the total number of tuples present.
Data Independence in the Relational Model
The relational model provides:
- Logical Data Independence: Changes in the conceptual schema (e.g., adding new attributes) do not affect the SQL queries, provided the changes are compatible.
- Physical Data Independence: Changes in the physical storage of data (e.g., indexing or file structures) do not impact the SQL queries, as the relational model abstracts physical storage details from the user.
Candidate Key, Primary Key, and Superkey
- Candidate Key: A minimal set of attributes that uniquely identify a tuple.
- Primary Key: A chosen candidate key used as the unique identifier.
- Superkey: A set of attributes that uniquely identifies tuples but may include extra attributes.
Explanation (300 words)
The relational model simplifies data representation by abstracting logical and physical aspects of data storage. A relation schema describes the structure of a table, while a relational database schema provides a blueprint for an entire database. Attributes in schemas have domains that define permissible values, ensuring data integrity. The number of rows in a relation instance is its cardinality, while the number of columns defines its degree.
Cardinality directly measures distinct tuples. For a relation instance with a cardinality of 22, there are 22 distinct tuples by definition.
A key feature of the relational model is data independence. Logical data independence ensures SQL queries remain unaffected by schema changes, such as adding attributes. Physical data independence allows database administrators to optimize storage mechanisms without modifying how queries are written or data is accessed, empowering seamless scalability and flexibility.
Keys in the relational model serve to enforce uniqueness and identify tuples. A candidate key is a minimal, unique identifier for tuples. The primary key is a selected candidate key that serves as the table’s main identifier. A superkey is any attribute set that can uniquely identify tuples, though it might include redundant attributes. For example, in a Student table with attributes ID and Name, {ID} could be the primary key, and {ID, Name} would be a superkey. The ability to define and enforce keys ensures data consistency and prevents redundancy.