Data Management (Foundations) D426

Correct Answer

B. view

Explanation

A view in a relational database is a virtual table that can restrict access to specific rows or columns of a database. It is created by defining a query that selects data from one or more tables, and it allows users to access only the subset of data that the view is designed to show. This provides a way to control access to sensitive information while maintaining a streamlined interface for users.

Why other options are wrong

A. primary key

A primary key is a column (or set of columns) in a database table that uniquely identifies each record in that table. While it is crucial for ensuring data integrity, it does not provide restricted access to certain rows or columns, as its role is to uniquely identify records within a table.

C. relation

A relation in database terminology refers to a table itself, or a set of tuples (rows). It does not inherently control access to specific rows or columns. Access restrictions are typically managed through views or user permissions, not through the concept of a relation.

D. tuple

A tuple is a single row in a database table, consisting of a set of values corresponding to the columns in the table. It does not control access or limit visibility of data. Access control is not the purpose of a tuple.

Correct Answer

A. A view for which data is stored and needs to be refreshed when base tables change

Explanation

A materialized view is a database object that stores the results of a query as actual data, rather than just the SQL statement. The data is physically stored and needs to be refreshed periodically or when the underlying base tables change. This differs from a regular view, which is virtual and does not store data, only the query definition.

Why other options are wrong

B. Virtual table created as a result of a SELECT query

This describes a regular view, not a materialized view. A regular view does not store data; it only stores the SQL query used to generate the result, and the data is re-computed every time the view is accessed.

C. A view that cannot be updated or altered

This is incorrect because a materialized view can be updated and altered, though the process of refreshing the data in the view is separate from altering the view itself. The statement does not accurately describe materialized views.

D. A view that only displays the most recent data

While materialized views do need refreshing to display the most current data, they are not restricted to showing only the most recent data. The view can be refreshed on a schedule or when triggered, but it does not automatically display only the most recent data unless specifically managed that way.

Correct Answer

A. From similar entities with many common attributes and relationships

Explanation

Supertype and subtype entities are typically created when there are similar entities with many common attributes and relationships. A supertype entity represents a common set of attributes, while subtypes represent specialized versions of the supertype. This is useful for reducing redundancy and making the database design more efficient. The relationships between these entities are often hierarchical.

Why other options are wrong

B. When entities have no common attributes

This is incorrect because supertype and subtype entities are used when there are common attributes between entities. If there are no common attributes, there would be no reason to create a supertype/subtype relationship.

C. When entities need to be completely separated in a database

This is incorrect because the goal of using supertype and subtype entities is to group similar entities together, not to completely separate them.

D. From entities that require exclusive and unique attributes

This is incorrect because supertype and subtype relationships are used to capture commonality, not to separate entities based on exclusivity or uniqueness of attributes.

Correct Answer:

Junction tables cannot have a concatenated primary key.

Explanation:

Junction tables are used to resolve many-to-many relationships by creating two one-to-many relationships. They typically include foreign keys referencing the related tables, and these foreign keys are often combined into a composite (concatenated) primary key to ensure uniqueness. Therefore, saying that junction tables cannot have a concatenated primary key is incorrect. In fact, concatenated primary keys are a common and valid practice in junction table design.

Why Other Options Are Wrong:

A junction table must contain the primary key from each of the related tables.

This is true because junction tables rely on foreign keys that reference the primary keys of the tables they link. This design ensures proper relationships and maintains referential integrity.

Each field is a foreign key that links back to the primary key in the related table.

This is correct. The junction table uses foreign keys to connect rows from two or more tables. These keys create the linkage that transforms many-to-many relationships into manageable one-to-many links.

Junction tables change the many-to-many relationship into two one-to-many relationships.

This statement is true because the main purpose of a junction table is to normalize a many-to-many relationship into two one-to-many relationships. This design prevents redundancy and improves consistency.

Correct Answer

B. Primary Key

Explanation

A Primary Key is a candidate key chosen to uniquely identify each row in a database table. It must contain unique values, and it cannot contain NULLs. The Primary Key ensures that each record in the table is uniquely identifiable.

Why other options are wrong

A. Alternate Key

An Alternate Key is any candidate key that is not selected as the Primary Key. It can also uniquely identify rows, but it is not chosen to be the Primary Key.

C. Foreign Key

A Foreign Key is a column or a set of columns in one table that links to the Primary Key or another unique key in another table. It is used to establish relationships between tables, not to uniquely identify records within a table.

D. None of the above

This option is incorrect because the correct answer is "Primary Key," which is explicitly listed as an option.

Correct Answer

A. UPDATE PRODUCTS SET PRICE = 5.45 WHERE (PROD_ID = '24');

Explanation

The correct SQL syntax to update an existing table involves the UPDATE command, followed by the table name (PRODUCTS), the SET clause to specify the column and the new value (PRICE = 5.45), and the WHERE clause to define the condition for which rows should be updated (PROD_ID = '24').

Why other options are wrong

B. SELECT PRODUCTS WHERE (PROD_ID = '24') SET PRICE = 5.45;

This is incorrect because the SELECT statement is used for querying data, not updating it. The UPDATE command is required to modify data in a table.

C. SELECT PRODUCTS UPDATE PRICE = 5.45 WHERE (PROD_ID = '24');

Again, this is incorrect because SELECT should not be used in an update query. The UPDATE statement must be used for modifying data.

D. UPDATE PROD_ID = '24' SET PRICE = 5.45;

This is incorrect because the UPDATE statement must specify the table name (e.g., UPDATE PRODUCTS), and conditions like WHERE should be used to specify which rows to update.

Correct Answer

A. They should be simple, easy to type, and quick to process in queries

Explanation

Primary key values should be simple, easy to type, and efficient for processing in queries. This ensures that indexing and lookups are fast, contributing to better performance in the database. Complex primary key values or long strings can slow down query performance and make it harder for users to work with the data.

Why other options are wrong

B. They should be able to store large amounts of data

This is incorrect. Primary keys should not store large amounts of data. They are typically small, simple values like integers or short strings that are easy to index and quick to search.

C. They need to be long strings for uniqueness

This is incorrect. While uniqueness is important, primary keys should not necessarily be long strings. Simple numeric or short alphanumeric keys are sufficient for ensuring uniqueness and are more efficient for indexing and querying.

D. They should contain complex data structures

This is incorrect. Complex data structures are not recommended for primary keys. Simple and efficient values ensure better performance in querying and indexing.

Correct Answer:

Create a composite key through a formula field to be used as a primary key.

Explanation:

When no single field uniquely identifies each row in a dataset, combining multiple columns into a composite key ensures uniqueness. A composite key is formed by using two or more fields together to create a unique identifier. This approach aligns with relational database design principles, ensuring data integrity while allowing the table to maintain proper indexing and referencing. It avoids guesswork or external tools and follows accepted database normalization practices.

Why Other Options Are Wrong:

Add a randomly generated number for each line and use machine learning to help identify key matches.

Randomly generated numbers can introduce inconsistencies and are not tied to the inherent data. Machine learning is unnecessary for simple relational database design and does not guarantee consistent uniqueness. This option overcomplicates the problem and does not follow standard practices.

Remove this data as it is not useful.

Discarding data simply because it lacks a single natural key is wasteful. Many valuable datasets can still be used effectively by creating composite keys. Eliminating data disregards possible insights and is not a valid database design solution.

Use Machine Learning to identify what should be the primary key.

Machine learning is not designed to identify relational primary keys. It may find patterns, but it cannot enforce strict database-level uniqueness. Using ML for this purpose is impractical, inefficient, and contrary to established database design methods.

Correct Answer

A. ALTER

Explanation

The ALTER command is used to modify the structure of an existing database schema. This can include altering tables, adding or dropping columns, modifying data types, or renaming tables and columns. It is a DDL (Data Definition Language) command that changes the definition of database objects.

Why other options are wrong

B. SELECT

The SELECT command is used to query data from a database, not to define or modify the structure of the schema. It retrieves data but does not alter the schema itself.

C. INSERT

The INSERT command is used to add new rows of data into a table. It does not affect the structure of the database schema, only the data within it.

D. EXECUTE

The EXECUTE command is used to execute a stored procedure or SQL query. It does not define or modify the schema, but rather runs predefined commands or statements.