Questions and Answers
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 is the result of mapping an ER schema?
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?
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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?
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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?
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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.
