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Questions and Answers
What is a foreign key in a relational database?
When the primary key of one relation appears as an attribute in another relation.
How is the primary key attribute represented in a relational schema?
The primary key attributes are underlined.
What is the purpose of a dashed underline in a relational schema?
To represent foreign key attributes.
What are the two principal integrity rules in the relational data model?
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How do nulls relate to integrity constraints in the relational model?
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Study Notes
Properties of Relations
- Rearranging rows in a relation does not change the relation.
- Each cell in a relation should contain at most one value.
- Records in a relation should be distinct, with no duplicates.
- Database systems may not enforce this property, allowing duplicate records.
Keys
- A key is a chosen attribute or set of attributes that uniquely identify each row in a relation.
- Keys enable retrieving or relating rows to other records.
- Keys must be minimal, with no superfluous attributes.
Basic Terminology
- Degree of a relation: the number of attributes it contains.
- Cardinality of a relation: the number of records it contains.
- A relational database is a collection of normalized relations.
Summary of Corresponding Terms
- Relation = Table = File
- Tuple = Row = Record
- Attribute = Column = Field
Properties of Relations
- Each relation has a unique name.
- Attribute names are unique only within their relation.
- Values of an attribute come from the same domain.
- Order of attributes and rows in a relation has no significance.
Relational Database Schemas
- Notation for each relation:
relation_name (attribute1, attribute2, …, attributeN) - Primary key attributes are underlined, and foreign key attributes have a distinguishing feature.
Relational Integrity
- Two principal integrity rules: entity integrity rule and referential integrity rule.
- Both rules depend on the concept of nulls, which represent unknown or inapplicable values.
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Description
Learn about the properties of relations in databases, such as the uniqueness of records and the limitation of values per cell. Understand why rearranging rows does not alter the essence of a relation.
