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Questions and Answers
What is the set of allowed values for each attribute called?
domain
Attribute values are always required to be atomic.
True
What does the null value indicate in a database?
unknown
Which of the following is a candidate key in the instructor relation?
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What is the primary logical structure of the database referred to as?
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Match the following relational algebra operations to their corresponding symbols:
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Order of tuples in a relation is relevant.
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Study Notes
Relational Model:
- Relational Databases are structured by representing data using tables (relations), with rows (tuples) and columns (attributes).
- Attribute Domain: The set of possible values for an attribute.
- Atomic Attributes: Attribute values are indivisible, meaning they cannot be broken down into smaller parts.
- Null Value: A special value indicating the attribute's value is unknown.
- Order of Tuples: Does not matter - tuples can be stored in any sequence.
- Database Schema: The logical structure of a database, outlining the relationships between tables, attributes, and constraints.
- Database Instance: A snapshot of the data stored in the database at a specific point in time.
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Keys: Used to uniquely identify tuples within a relation:
- Superkey: A set of attributes that can identify a unique tuple within a relation.
- Candidate Key: Minimal superkey - a set of attributes that can identify a unique tuple without any unnecessary attributes.
- Primary Key: One chosen candidate key that is used to uniquely identify tuples.
- Foreign Key Constraint: A constraint that ensures the value in one table's attribute (foreign key) matches a value in another table's attribute (primary key), establishing a relationship between the tables.
- Schema Diagram: A visual representation of the database schema, showing tables and relationships using symbols.
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Relational Query Languages: Used to query and manipulate data in a relational database:
- Procedural: Provide step-by-step instructions for data manipulation.
- Non-procedural (Declarative): Focus on what data is needed, not how to retrieve it.
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Pure Relational Languages
- Relational Algebra: A procedural language for database manipulation.
- Tuple Relational Calculus: A non-procedural language based on tuples.
- Domain Relational Calculus: Another non-procedural language based on domains.
- All three languages are equivalent in power, meaning they can express the same queries.
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Relational Algebra Operators:
- Select (σ): Extracts rows matching a given condition.
- Project (∏): Extracts specific columns (attributes).
- Union (∪): Combines rows from two relations with identical attributes.
- Set Difference (-): Removes rows from one relation that are also present in another.
- Cartesian Product (x): Combines every row from one relation with every row from another.
- Rename (ρ): Changes attribute names or table names.
Relational Algebra Operations:
- Select Operation (σ): Used for extracting specific rows from a relation based on a condition.
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Description
This quiz covers the fundamental concepts of relational databases, including attributes, tuples, schemas, and keys. Test your understanding of key terms such as atomic attributes, null values, and the significance of database instances. Perfect for learners seeking to grasp the relational model structure.
