22 Questions
What is a primary key in a relation?
A combination of attributes that uniquely identifies each row in a relation
How is a primary key designated in a relation?
By underlining the attribute name(s)
What does a relation in a relational database model represent?
A named, two-dimensional table of data
How is the structure of a relation expressed using shorthand notation?
By placing attribute names in parentheses
What is an ER diagram used for in database design?
To represent the graphical view of entities and their relationships
In the relational model, what is a relation composed of?
Named columns and arbitrary number of unnamed rows
What is a Data Dictionary in the context of database systems?
A collection of metadata that describes the data objects in the database.
What is the purpose of a System Catalog in a database system?
To store metadata about the database objects such as tables, views, and indexes.
Which of the following statements is true about a Data Dictionary?
It helps in ensuring data integrity and consistency.
In a database system, what role does the System Catalog play during query processing?
It provides metadata needed to validate and process queries.
How does a Data Dictionary differ from a System Catalog in a relational database management system?
A System Catalog contains detailed information about database objects, while a Data Dictionary does not.
Which of the following best describes the primary function of a System Catalog in a database system?
To manage and store metadata related to database schema and objects.
What is the purpose of a data dictionary in a database system?
To contain metadata about the tables in the database
Which function does the system catalog serve in a database system?
It provides a detailed system data dictionary
What does the term 'homonym' refer to in the context of database attribute names?
Using the same attribute name for different meanings
How does an outer join differ from an inner join?
Outer join retains unmatched values from one table
In a database system, what is the purpose of an equijoin?
To link tables based on specified column equality
Which type of index ensures that each index key has only one associated row in a table?
Unique index
What is the main function of an index key in a database system?
To locate the row associated with each index key value
When using an equijoin, how are tables linked together?
By selecting rows with common values in specific attributes
What is an example of a synonym in a database system?
CAR and AUTO
Why is data redundancy sometimes considered necessary?
To ensure transaction speed and information requirements are met
Study Notes
Relational Database Models
- A relational database model represents data in the form of tables based on mathematical theory.
- A relation is a named, two-dimensional table of data, consisting of a set of named columns and an arbitrary number of unnamed rows.
Relational Algebra
- Relational algebra defines a theoretical way of manipulating table contents using relational operators.
- Relational algebra operators on existing relations produce new relations.
- Relational algebra operators include:
- SELECT: yields all values for all rows in a table that satisfy a given condition.
- PROJECT: yields all values for selected attributes - a vertical subset of a table.
- JOIN: combines all rows from two tables, excluding duplicate rows.
- UNION: yields all rows in one table that are not found in the other table.
- DIFFERENCE: yields all rows in one table that are not found in the other table.
- PRODUCT (or CARTESIAN PRODUCT): yields all possible combinations of rows from two tables.
- DIVIDE: uses one 2-column table as the dividend and one single-column table as the divisor.
Relational Set Operators
- Natural join: links tables by selecting rows with common values in common attributes (join columns).
- Equijoin: links tables on the basis of an equality condition that compares specified columns.
- Theta join: a comparison operator other than equal is used.
- Inner join: only returns matched records from the tables that are being joined.
- Outer join: returns all matched records (as an inner join) but returns the unmatched records from one of the tables.
- Left outer join: yields all of the rows in the CUSTOMER table, including those that do not have a matching value in the AGENT table.
- Right outer join: yields all of the rows in the AGENT table, including those that do not have matching values in the CUSTOMER table.
Data Dictionary and System Catalog
- Data dictionary: provides detailed accounting of all tables found within the user/designer-created database.
- System catalog: contains metadata and detailed system data dictionary that describes all objects within the database.
Homonyms and Synonyms
- Homonym: indicates the use of the same name to label different attributes.
- Synonym: indicates the use of different names to describe the same attribute.
Data Redundancy and Indexes
- Data redundancy: leads to data anomalies and can destroy the effectiveness of the database.
- Foreign keys: control data redundancies by using common attributes shared by tables.
- Indexes: an orderly arrangement to logically access rows in a table.
- Index key: the index's reference point that points to data location identified by the key.
- Unique index: an index in which the index key can have only one pointer value (row) associated with it.
Test your knowledge of SQL joins and relational algebra with this quiz! Questions cover topics like SELECT, PROJECT, JOIN, INTERSECT, UNION, and more from Chapter 3 of relational database models.
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