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Questions and Answers
What is the process of defining a set of subclasses of a superclass called?
What is the process of defining a set of subclasses of a superclass called?
What type of constraint applies to a specialization/generalization where every subclass has only one superclass?
What type of constraint applies to a specialization/generalization where every subclass has only one superclass?
What is the result of mapping an ER schema?
What is the result of mapping an ER schema?
What is the process of creating a superclass from a set of subclasses called?
What is the process of creating a superclass from a set of subclasses called?
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What type of constraint applies to a specialization/generalization where a subclass can be a subclass of more than one superclass?
What type of constraint applies to a specialization/generalization where a subclass can be a subclass of more than one superclass?
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What is the step in ER-to-Relational mapping that involves mapping of superclass/subclass relationships?
What is the step in ER-to-Relational mapping that involves mapping of superclass/subclass relationships?
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What is the term for a subclass that inherits all attributes of the entity as a member of the superclass?
What is the term for a subclass that inherits all attributes of the entity as a member of the superclass?
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What type of relationship is formed when a subclass is a subclass of more than one superclass?
What type of relationship is formed when a subclass is a subclass of more than one superclass?
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What is the term for a subclass that has further subclasses specified on it?
What is the term for a subclass that has further subclasses specified on it?
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What is the purpose of ER-to-Relational mapping?
What is the purpose of ER-to-Relational mapping?
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Study Notes
Relational Algebra
- Enables users to specify basic retrieval requests
- Operations produce new relations that can be further manipulated
- Fundamental operations: • Union (∪) • Set difference (-) • Selection (σ) • Projection (π) • Cartesian product (X)
- Additional operations: • Rename (ρ) • Intersection (∩) • Join • Division (÷)
- Type compatibility requirements: • Same degree • Corresponding attributes defined over the same domain
SQL
- Language for expressing queries on relations
- Based on relational algebra for sets and bags
- Supports creation and modification of relations
- Examples of SQL queries: • CREATE TABLE R () • INSERT INTO R VALUES (v1; … ; vn) • DELETE FROM R WHERE C • UPDATE R SET A = v WHERE C • SELECT .. FROM … WHERE…
Relations
- A two-dimensional table
- Attributes ≈ column names
- Tuples ≈ rows (not including header row)
- Database ≈ collection of relations
- Relation characteristics: • Each relation has a distinct name • Each attribute has a distinct name • Values of an attribute are from the same domain • Each tuple is distinct • Entity degree is the number of fields/attributes in schema • Entity cardinality is the number of tuples in relation
Relational Model Integrity Constraints
- Conditions that must hold on all valid relation instances
- Three main types: • Key constraints • Entity integrity constraints • Referential integrity constraints • Semantic integrity constraints
- Specified when schema is defined
- Checked when relations are modified
Relational Data Model Operations
- Two categories: • Retrieval operations (extract information) • Update operations (cause relation state changes)
EERD (Enhanced ER or Extended ER)
- EER diagrams extend ER diagrams to represent additional subgroupings
- Subclasses or subtypes
- Superclass/subclass relationships
- Inheritance: • All attributes of the superclass • All relationships of the superclass
- Specialization: defining subclasses of a superclass
- Generalization: reverse of specialization process
Constraints on Specialization and Generalization
- Three basic constraints: • Condition-Defined Constraint • Disjointness Constraint • Completeness Constraint
Lattices & Shared Subclasses
- A subclass may have further subclasses specified on it
- Forms a hierarchy or lattice
- Hierarchy: every subclass has only one superclass (single inheritance)
- Lattice: a subclass can be subclass of more than one superclass (multiple inheritance)
ER-to-Relational Mapping
- Step 1: Mapping of Regular Entity Types
- Step 2: Mapping of Weak Entity Types
- Step 3: Mapping of Binary 1:1 Relation Types
- Step 4: Mapping of Binary 1:N Relationship Types
- Step 5: Mapping of Binary M:N Relationship Types
- Step 6: Mapping of Multi-valued attributes
- Step 7: Mapping of N-ary Relationship Types
- Step 8: Mapping Super class/ Sub class relationship
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Description
Learn about relational algebra operations in systems analysis and design, including retrieval requests and relation manipulation. Based on Dennis, Wixom, & Roth's 3rd Edition.
