Question 1

In entity–relationship (ER) modeling, evaluate the following definition and choose whether it is correct or not.  “An ID-dependent entity is an entity whose identifier is a composite identifier where no portion of the composite identifier is an iden…

Accepted Answer

Introduction / Context: ID-dependent (also called weak) entities are a foundational topic in data modeling. The question asks whether an ID-dependent entity's key excludes any parent identifier. Understanding this distinction helps you model one-to-many ownership, enforce referential integrity, and create meaningful primary keys for intersection tables and weak entities. Given Data / Assumptions: We are using standard ER terminology: entity types, attributes, identifiers (keys), relationships. “ID-dependent entity” means an entity whose existence and identification depend on a parent entity. Composite identifier means a primary key made of two or more attributes. Concept / Approach: By definition, an ID-dependent entity has an identifier that includes the identifier of another (owner/parent) entity. Typical examples are OrderLine depending on Order, or Dependent depending on Employee. The key of the weak/ID-dependent entity “borrows” the parent’s key (fully or a unique part of it) and adds its own partial key (like line_no) to form a composite identifier. Step-by-Step Solution: State the standard rule: a weak or ID-dependent entity’s primary key includes the parent’s primary key.Compare with the statement: it claims that no portion of the composite identifier is an identifier of another entity.This directly contradicts the rule, because the composite key must contain the parent’s key.Therefore, the definition given is incorrect. Verification / Alternative check: Consider Order(order_id) and OrderLine(order_id, line_no). OrderLine is ID-dependent; its PK contains order_id, which is part of the parent’s identifier. This falsifies the given statement. Why Other Options Are Wrong: “Valid statement” contradicts the standard definition. “Sometimes valid only in weak-entity diagrams” is self-contradictory because weak entities are exactly where parent keys are included. “Ambiguous” is unnecessary—this is a clear definitional point. “Not applicable” is irrelevant; the concept is core to ER modeling. Common Pitfalls: Confusing composite identifiers in general with ID-dependence; assuming “composite” implies total independence from any parent. Also, mixing up identifying vs. non-identifying relationships. Final Answer: Invalid statement

Question 2

In database design workflow, is the following sequencing a sound practice? First identify entities, then determine their attributes, and finally establish the relationships among those entities.

Accepted Answer

Introduction / Context: Database design typically progresses from high-level understanding to detailed structure. A common approach is to: identify entities, list attributes, and then capture relationships. The question asks whether this sequence is sound as a general guideline in conceptual and logical modeling. Given Data / Assumptions: We are discussing early-stage (conceptual/logical) modeling, not vendor-specific physical tuning. Entities represent real-world things or concepts users need to track. Attributes describe properties of those entities. Relationships connect entities and define cardinality and optionality. Concept / Approach: Starting with entities reduces noise and keeps focus on business nouns. Attributes attached to entities give concrete structure. Once entities and their attributes are clear, relationships can be defined with appropriate cardinalities (1:1, 1:N, M:N), sometimes introducing associative entities for M:N. This sequence is widely taught and used, even though iterative refinement is expected. Step-by-Step Solution: Elicit business requirements to list candidate entities.For each entity, enumerate attributes and candidate identifiers.Define relationships (including participation and cardinalities) and resolve M:N via associative entities if needed.Iterate with normalization to remove anomalies. Verification / Alternative check: Cross-check with standard methodologies (e.g., ER modeling texts): entities → attributes → relationships is a common, effective starting order, with iterative feedback loops for normalization and constraint validation. Why Other Options Are Wrong: “Invalid” contradicts common practice. “Only for physical design” is incorrect; this is conceptual/logical. “Ambiguous — DBMS” confuses logical modeling with physical implementation. “Only after normalization” reverses the process; normalization follows initial structure. Common Pitfalls: Locking the sequence rigidly; in practice, you iterate. Also, jumping to relationships first can lead to weak entity definitions. Final Answer: Valid statement

Question 3

Clarify the definition of a ternary relationship in ER modeling: does “ternary” mean two entities plus one association, or something else?

Accepted Answer

Introduction / Context: Relationship degree is a precise term in ER modeling. Unary (degree 1) involves one entity type (recursive relationship), binary (degree 2) involves two, and ternary (degree 3) involves three. The question challenges a common misunderstanding of “ternary.” Given Data / Assumptions: We are modeling at the conceptual level with standard ER notation. “Association” refers to the relationship itself connecting entities. Concept / Approach: A ternary relationship connects three entity types simultaneously in a single relationship instance. Its semantics cannot generally be decomposed into independent binary relationships without potentially losing constraints. For example, Supplier–Part–Project as a ternary “supplies” relationship captures which supplier supplies which part to which project at once. Step-by-Step Solution: Define relationship degree: number of entity types participating.Map “ternary” to degree 3 → requires three entity types.Evaluate the statement claiming “two entities and one association.”Conclude it is incorrect; two entities form a binary relationship, not a ternary one. Verification / Alternative check: Attempt to model supplier–part–project as three binaries; you will not enforce that the same supplier–part pair applies to a specific project without additional constraints, hence the need for a ternary relationship. Why Other Options Are Wrong: “Valid” is false; “recursive” does not change the degree count; “physical design” is unrelated; “ambiguous” is unnecessary—degree has a clear definition. Common Pitfalls: Assuming any relationship can be decomposed into binaries without loss; miscounting degree when one entity appears twice (that is still unary or binary depending on context). Final Answer: Invalid statement — a ternary relationship involves three entity types in one relationship

Question 4

In a 1:N (one-to-many) relationship, identify which side is the parent and which is the child according to ER modeling conventions.

Accepted Answer

Introduction / Context: Correctly naming the “parent” and “child” in a 1:N relationship is important for understanding ownership, foreign keys, and the direction of optionality. This convention is used in ER diagrams and relational schema design. Given Data / Assumptions: We consider classic 1:N relationships such as Customer–Order, Department–Employee. Parent supplies the primary key that the child references via a foreign key. We are not limiting to identifying vs. non-identifying relationships; the convention applies to both. Concept / Approach: In 1:N, the entity on the one (1) side is the parent; each parent may relate to many children. The entity on the many (N) side is the child, which carries the foreign key back to the parent. Identifying (solid line) relationships embed the parent PK into the child’s PK, while non-identifying (dashed) place it as a non-key attribute—yet the parent/child terminology remains the same. Step-by-Step Solution: State the convention: 1-side = parent; N-side = child.Apply to examples: Customer (parent) → Order (child); Department (parent) → Employee (child).Confirm that the child holds the FK to the parent’s PK.Therefore, the given statement is correct. Verification / Alternative check: Examine schema DDL: orders(customer_id FK references customers(id)). The FK lives in the N-side child, matching the convention. Why Other Options Are Wrong: Options reversing parent/child contradict schema practice; denying the convention ignores standard vocabulary; tying parentage solely to identifying relationships is incorrect. Common Pitfalls: Confusing optionality with cardinality; believing nullable FK flips parent/child roles; assuming parent must be mandatory in all cases. Final Answer: Valid statement — parent is on the 1 side; child is on the N side

Question 5

Definition check: In ER modeling, an entity is anything identifiable in the user’s work environment that users want to track. Is this characterization correct?

Accepted Answer

Introduction / Context: Defining “entity” precisely is the first step in conceptual database design. Entities represent classes of real-world objects or concepts about which the organization needs to store information—things users care to track. The question asks whether this common definition is correct. Given Data / Assumptions: We are at the conceptual ER level (entity types, attributes, identifiers). “User’s work environment” refers to the business domain being modeled. Entities become relations/tables in the relational model. Concept / Approach: Entities correspond to nouns: Customer, Product, Invoice, Project, etc. An entity type defines attributes (name, identifier, dates) and constraints; entity instances are the individual occurrences. If users do not need to track it, it is not an entity (or not in scope). Conversely, if something is important to track, it is a candidate entity. Step-by-Step Solution: Restate the definition: an entity is an identifiable, distinguishable thing in the problem domain.Check alignment: users want to track it → it belongs in the model.Map to schema: entities typically map to tables (possibly multiple tables when normalized).Therefore, the provided characterization is correct. Verification / Alternative check: Create a quick noun list from requirements; most nouns become entities or attributes. Those requiring independent existence and multiple attributes become entity types. Why Other Options Are Wrong: Declaring the statement “invalid” contradicts standard textbooks. Limiting to physical tables ignores conceptual design. Calling only processes entities confuses entities with activities/events (which can be entities too, but not exclusively). Requiring single-attribute keys is unrelated to the definition. Common Pitfalls: Over-scoping by turning every noun into an entity; under-scoping by burying distinct things as mere attributes. Final Answer: Valid statement

Question 6

Primary keys in practice: does an identifier “typically” use more than one attribute (i.e., is a composite key the usual case)?

Accepted Answer

Introduction / Context: Designers often debate natural vs. surrogate keys and single-attribute vs. composite identifiers. The question claims that identifiers “typically” use multiple attributes. Understanding common practice helps produce simpler joins and clearer foreign keys. Given Data / Assumptions: We are discussing general practice, not a special domain with unavoidable composites. Identifiers (primary keys) should uniquely identify entity instances. Surrogate keys (e.g., IDENTITY/sequence/UUID) are widely used. Concept / Approach: While composite identifiers exist (especially in associative/bridge tables or when using natural keys that require multiple columns), many schemas prefer a single-attribute primary key (often surrogate) for simplicity. Foreign keys referencing a single column ease indexing and joining, and surrogate keys avoid change-propagation when business values mutate. Step-by-Step Solution: Assess the “typical” case across common systems: single-column integer/UUID PKs are prevalent.Recognize exceptions: associative entities often have composite PKs of the two parent FKs.Therefore, the statement “typically uses more than one attribute” is inaccurate as a general rule. Verification / Alternative check: Survey popular schemas (ERP, CRM); most base tables use single-column PKs. Composite keys are concentrated in intersection tables or domains with strict natural keys. Why Other Options Are Wrong: Asserting composites are “most” common overgeneralizes. Tying the claim to 3NF is unrelated—normalization does not force composite PKs. Surrogate keys are single-column by nature. Common Pitfalls: Using wide composite PKs everywhere, complicating FKs and indexes; alternatively, overusing surrogates without enforcing natural-key uniqueness with alternate keys. Final Answer: Invalid statement — single-attribute identifiers are more typical

Question 7

ER terminology check: are “relationship classes” correctly described as associations between entity classes (types)?

Accepted Answer

Introduction / Context: Different notations use slightly different words: ER modeling speaks of entity types and relationship types (sometimes called “classes”), while UML class diagrams refer to classes and associations. The question asks whether describing relationship classes as associations between entity classes is correct. Given Data / Assumptions: We treat “relationship class/type” as the schema-level definition of how entities relate. We treat “entity class/type” as the schema-level definition of an entity. Concept / Approach: At the conceptual level, a relationship type specifies a named association between participating entity types, together with roles, cardinalities, and optionalities. This mirrors UML’s notion of an association between classes. Therefore, saying “relationship classes are associations between entity classes” is an accurate description across modeling vocabularies. Step-by-Step Solution: Map ER → relationship type; UML → association.Recognize that both connect two or more classifiers (entity types/classes).Conclude the statement faithfully describes the concept. Verification / Alternative check: Compare an ER diagram (Customer–places–Order) with a UML association between Customer and Order; both express the same structural link plus multiplicity. Why Other Options Are Wrong: Limiting correctness to recursive or 1:1 cases ignores that relationships of any cardinality/degree are associations. Claiming it only applies to UML is unnecessary; the notion is shared. Common Pitfalls: Confusing relationship type with relationship instance; mixing UML and ER notation without aligning terms. Final Answer: Valid statement

Question 8

At the instance level, are entity occurrences connected to each other by relationship classes (types) defined in the schema?

Accepted Answer

Introduction / Context: ER models distinguish between schema-level definitions (entity types and relationship types/classes) and data-level occurrences (entity instances and relationship instances). Understanding this mapping clarifies how actual rows are linked in a database. Given Data / Assumptions: Relationship classes (types) describe allowed associations between entity classes (types). For each relationship type, many relationship instances may exist at runtime. Relationally, relationship instances are represented via foreign keys or associative tables. Concept / Approach: Yes—entity instances are associated through relationship instances that conform to relationship classes. For a binary 1:N, the N-side rows carry a foreign key referencing the 1-side; for M:N, an associative (bridge) table holds pairs of foreign keys. All of these operational links are manifestations of the relationship class defined in the schema. Step-by-Step Solution: Define relationship class/type at schema level.Observe data level: each actual link (e.g., an order belonging to a customer) is a relationship instance.Therefore, entity instances are connected by instances of those classes, not ad-hoc. Verification / Alternative check: Check sample data: orders.customer_id references customers.id. This FK encodes relationship instances, per the relationship class “Customer places Order.” Why Other Options Are Wrong: Limiting to ternary is arbitrary; dismissing relationship types ignores model semantics; nullability is unrelated to whether instances are associated. Common Pitfalls: Equating foreign keys with attributes rather than recognizing them as relationship encodings; assuming relationships only exist at design time. Final Answer: Valid statement

Question 9

Composite identifiers: confirm whether a composite primary key must consist of two or more attributes.

Accepted Answer

Introduction / Context: Composite identifiers (composite primary keys) are common in associative entities and certain natural-key domains. The question asks whether the definition “two or more attributes form the identifier” is correct. Given Data / Assumptions: Primary key uniquely identifies each row/instance. Composite means the key is formed by multiple attributes (columns). No assumption is made that the attributes must be foreign keys, though that is common in bridge tables. Concept / Approach: A composite identifier is precisely a key consisting of two or more attributes. Typical examples include many-to-many bridge tables (e.g., student_course(student_id, course_id)) or natural keys like country_code + local_number for phone numbers in certain systems. Step-by-Step Solution: Define composite: more than one attribute participates in uniqueness.Check examples: dual FK bridges, date + sequence combinations, code + version pairs.Therefore, the statement matches the formal definition. Verification / Alternative check: Examine DDL: PRIMARY KEY (student_id, course_id)—this is a composite identifier of two attributes. Why Other Options Are Wrong: Requiring both attributes to be FKs is unnecessary; normalization choices do not change the definition; surrogate keys are single columns and are unrelated to whether alternates are composite. Common Pitfalls: Assuming composite keys are always inferior; in associative tables they express natural uniqueness succinctly. Conversely, using composites where a surrogate would be simpler can complicate FKs. Final Answer: Valid statement

Question 10

Schema consistency: do all instances (occurrences) of a given entity class share the same set of attributes (though values may be null or optional)?

Accepted Answer

Introduction / Context: In ER and relational modeling, an entity class (type) defines a fixed schema: a set of attributes and constraints that every instance obeys. The question asks whether each instance of the same entity class has the same attributes. Given Data / Assumptions: We are dealing with classic relational/ER modeling, not schemaless NoSQL. Optional attributes may be null for some instances. Subtype/supertype hierarchies may split attributes across related entity types, but each specific entity type still has a fixed attribute set. Concept / Approach: Yes—the definition of an entity class/type includes its attributes. Every instance conforms to that schema. Optionality affects whether a value is present, not whether the attribute exists. In relational terms, all rows of a table have the same columns; nullability governs missing values. Step-by-Step Solution: Define entity class/type: a template specifying attributes and constraints.Observe that instances are created according to that template; therefore, they share the attribute set.Account for optionality: attributes may be null, but they still exist for each instance.Therefore, the statement is correct. Verification / Alternative check: In SQL, CREATE TABLE defines columns (attributes); every row has those columns. Subtypes may be separate tables or single-table inheritance, yet each concrete type has a defined set of attributes. Why Other Options Are Wrong: Claiming invalidity ignores schema semantics; tying correctness to subtyping or storage engines confuses logical design with implementation; insisting on “no optional attributes” conflates presence of values with presence of attributes. Common Pitfalls: Assuming optional attributes are absent; misunderstanding inheritance—each subtype still has a defined schema. Final Answer: Valid statement

Question 11

In entity–relationship (ER) modeling, is “maximum cardinality” defined as the maximum number of entity instances that can participate in one occurrence of a given relationship?

Accepted Answer

Introduction / Context: Maximum cardinality is a foundational ER-modeling concept used to express how many entity instances may participate in a single relationship occurrence. Understanding it helps determine table structures, foreign keys, and constraints when transforming an ER model into a relational schema. Given Data / Assumptions: We are discussing classic ER modeling (Chen or Crow’s Foot style). “Relationship instance” means one specific occurrence of the relationship at a point in time. Entities are well-defined with identifiers; relationships connect instances from participating entity types. Concept / Approach: Cardinality describes participation counts. Maximum cardinality specifies the upper bound of how many instances on one side can be associated with a single instance on the other side in a relationship. Typical values are 1 or N (many). In Crow’s Foot, a short mark near an entity denotes “one,” and the three-pronged “crow’s foot” denotes “many.” Step-by-Step Solution: Identify the entities (e.g., Customer and Order). State the relationship (Customer places Order). Assess business rules: a Customer can place many Orders; each Order belongs to one Customer. Thus, maximum cardinalities are Customer: N on Orders; Order: 1 on Customer. This matches the definition that maximum cardinality = maximum number of instances allowed to participate on a side of a relationship instance. Verification / Alternative check: If business rules change (e.g., an order could belong to multiple customers), maximum cardinality would change accordingly (from 1 to N), confirming the concept is the defined upper bound, not a notation artifact. Why Other Options Are Wrong: “Incorrect” contradicts standard ER definitions. “Depends only on diagram notation” confuses visual syntax with the semantic rule. “Not applicable” is false because cardinality is central in ER models. Common Pitfalls: Confusing maximum with minimum cardinality (optionality), or assuming “many” always applies on both sides by default without analyzing business rules. Final Answer: Correct

Question 12

In conceptual and logical data modeling, do entities use identifiers while tables (relational implementations) use keys to uniquely distinguish records?

Accepted Answer

Introduction / Context: Models progress from conceptual (entities) to logical/physical (tables). The vocabulary shifts slightly: entities have identifiers; tables implement identifiers as keys (primary, candidate, alternate). Grasping the mapping ensures a clean translation from ER diagrams to relational schemas. Given Data / Assumptions: An “identifier” uniquely identifies an entity instance in the conceptual model. A “key” (primary key or candidate key) uniquely identifies a row in a table. We are talking about standard relational design conventions. Concept / Approach: The identifier of an entity becomes a table key when the model is implemented. For example, the entity Student may have StudentNumber as its identifier; in the relational schema, table STUDENT implements PRIMARY KEY (StudentNumber). Foreign keys implement inter-entity relationships. Step-by-Step Solution: Confirm that an entity requires an identifier to avoid ambiguity among instances. Map entities to tables and identifiers to keys. Define PKs and FKs accordingly (e.g., STUDENT(StudentNumber PK), ENROLMENT(StudentNumber FK)). Validate uniqueness and referential integrity in the DBMS. Verification / Alternative check: Review any normalization exercise: identifiers logically precede the key choice; the PK chosen enforces uniqueness of rows that represent entity instances. Why Other Options Are Wrong: “Incorrect” ignores standard terminology alignment. NoSQL and dimensional qualifiers are distractions; the statement concerns core relational mapping. Common Pitfalls: Treating business identifiers and surrogate keys as the same; a surrogate key is a technical key that may implement or supplement the conceptual identifier. Final Answer: Correct

Question 13

Are 1:1, 1:N, and N:M ER relationships “also known as HAS-A relationships,” or is “HAS-A” primarily an object-oriented association term rather than ER cardinality terminology?

Accepted Answer

Introduction / Context: ER modeling uses precise terms for relationships and cardinalities (1:1, 1:N, N:M). “HAS-A” is a colloquial phrase drawn from object-oriented design to describe aggregation or composition between classes. It is not a synonym for ER cardinality categories. Given Data / Assumptions: ER: relationships are characterized by degree, cardinality, and optionality. OO: “HAS-A” often implies ownership/aggregation between objects. Statement claims ER categories are “also known as HAS-A.” Concept / Approach: While both ER and OO describe associations, ER cardinality is formal database modeling terminology. “HAS-A” does not convey minimum/maximum participation or intersection entities required for N:M. Therefore, equating them is misleading and imprecise in database design contexts. Step-by-Step Solution: List ER types: 1:1, 1:N, N:M. Note what each specifies: maximum participation counts for each side. Contrast with “HAS-A,” which lacks explicit cardinality and optionality semantics. Conclude that “HAS-A” is not a correct alias for ER cardinality types. Verification / Alternative check: Consult standard ER references: you will find cardinality/degree/optionality but not “HAS-A” as a formal synonym. OO references use “HAS-A” for aggregation/composition semantics. Why Other Options Are Wrong: “Correct” or “composition-only” still mislabel ER categories. Notation choice (Crow’s Foot vs. Chen) does not redefine terminology. Common Pitfalls: Mixing OO vocabulary with ER modeling; assuming similar diagrams imply identical semantics and terms. Final Answer: Incorrect

Question 14

In ER modeling, does an attribute describe (capture) the characteristics or properties of an entity instance such as name, date, or amount?

Accepted Answer

Introduction / Context: Attributes are the named data elements that describe an entity’s characteristics (e.g., Customer.Name, OrderDate, Salary). Correctly modeling attributes is essential for normalization, constraints, and data quality rules. Given Data / Assumptions: Entities represent real-world things or concepts. Attributes hold values per entity instance. Identifiers are typically composed of one or more attributes. Concept / Approach: Attributes define what you store about each instance. They can be simple, composite, multi-valued (conceptually), or derived. During relational transformation, attributes become columns with data types, nullability, and constraints. Step-by-Step Solution: Pick an entity (e.g., Employee). List attributes (EmpID, Name, HireDate, Salary). Classify attributes: identifier vs. descriptive vs. derived. Map to table columns and specify types/constraints. Verification / Alternative check: Check whether each attribute is atomic and relevant to the entity; if not, consider new entities or normalization (e.g., split Address). Why Other Options Are Wrong: “Incorrect” contradicts the standard definition; restricting to weak entities or numeric data is unfounded. Common Pitfalls: Embedding multiple facts in one attribute; ignoring derived/calculated values vs. stored values; failing to enforce domain constraints. Final Answer: Correct

Question 15

Does the “degree” of a relationship indicate the number of participating entity types (for example, unary = 1, binary = 2, ternary = 3)?

Accepted Answer

Introduction / Context: Relationship degree is a classic ER concept distinct from cardinality. Degree counts how many entity types participate. Most operational schemas are built from binary relationships, but unary (recursive) and ternary (three-way) relationships occur in practice. Given Data / Assumptions: Unary: one entity type relates to itself (e.g., Employee manages Employee). Binary: two entity types (e.g., Customer places Order). Ternary: three entity types (e.g., Doctor prescribes Drug to Patient). Concept / Approach: Degree answers “how many entity types?” while cardinality answers “how many instances on each side?” and optionality answers “minimum participation required?” Keeping these distinct helps avoid modeling errors. Step-by-Step Solution: Identify participants in a relationship. Count the distinct entity types to determine degree. Separately specify maximum and minimum cardinalities for each participating side. Verification / Alternative check: Transform a ternary relationship into associative tables; verify that the relationship truly requires three-way dependency, not two separate binaries. Why Other Options Are Wrong: “Incorrect” conflicts with accepted definitions. Limiting to binary or tying to optionality mixes separate properties. Common Pitfalls: Forcing ternary relationships into binary pairs without preserving semantics; conflating degree with cardinality. Final Answer: Correct

Question 16

Does an entity’s identifier determine the “type” of relationship it has, or does the identifier simply provide uniqueness while relationship type is governed by business rules and cardinality?

Accepted Answer

Introduction / Context: An identifier uniquely distinguishes entity instances. It does not decide whether a relationship is 1:1, 1:N, or N:M. Relationship type depends on business semantics and constraints expressed as cardinality and optionality, not on how we uniquely label instances. Given Data / Assumptions: Identifiers exist to ensure each entity instance is uniquely referenceable. Relationships reflect how instances of entities are associated in the domain. Keys (surrogate or natural) are implementation choices for uniqueness. Concept / Approach: Decide relationship types by analyzing rules (e.g., “each Order must belong to one Customer” implies 1 on the Order side to Customer). The identifier does not change that determination. Whether the Customer identifier is CustomerNumber or a surrogate CustomerID is irrelevant to cardinality. Step-by-Step Solution: Gather business rules that dictate participation counts. Set cardinalities accordingly (max/min). Choose identifiers to uniquely tag instances; keep this separate from relationship analysis. Verification / Alternative check: Switch a natural key to a surrogate key in a model; the relationship types do not change, confirming independence from identifiers. Why Other Options Are Wrong: Claiming identifiers determine relationship type confuses uniqueness with association semantics. Weak-entity or surrogate-key scenarios still do not make identifiers dictate cardinality. Common Pitfalls: Letting technical key choices drive conceptual modeling; mixing normalization/key strategy with relationship semantics. Final Answer: Incorrect

Question 17

In ER modeling, is a recursive relationship defined as a relationship between instances of the same entity type (for example, Employee manages Employee), rather than between an “archetype” and an instance?

Accepted Answer

Introduction / Context: A recursive (unary) relationship relates an entity type to itself. Classic examples include employee-to-manager, part-to-subpart (bill of materials), and person-to-guardian. The notion of “archetype vs. instance” is not ER terminology for recursion and confuses separate modeling ideas. Given Data / Assumptions: Recursive = unary relationship involving a single entity type. Archetype/instance phrasing is not part of standard ER definitions. Cardinality and optionality still apply (e.g., each Employee may have zero or one manager; a manager may supervise many employees). Concept / Approach: Model recursion by relating the entity to itself with appropriate roles (e.g., Employee as “Manager” and Employee as “Subordinate”), and set min/max cardinalities per role. This is independent of inheritance (supertype/subtype) structures. Step-by-Step Solution: Select entity type (Employee). Create relationship EMPLOYEE manages EMPLOYEE with role names. Set cardinalities: each Employee may have 0..1 manager; a Manager may have 0..N subordinates. Implement with a self-referencing foreign key (e.g., Employee.ManagerID → Employee.EmployeeID). Verification / Alternative check: Check that removing the recursive FK breaks manager-subordinate traversal, confirming the self-relationship semantics. Why Other Options Are Wrong: Saying recursion is between an archetype and instance misuses terms; recursion is between two roles of the same entity type. Tying recursion to 1:1 or inheritance is also incorrect. Common Pitfalls: Forgetting to name roles; trying to create two separate entities instead of a single entity with a recursive relationship. Final Answer: Incorrect

Question 18

Is a subtype entity a special case (subset) of a more general supertype entity in supertype–subtype (inheritance) modeling?

Accepted Answer

Introduction / Context: Supertype–subtype modeling captures specialization and generalization. A subtype inherits the attributes and identifier of its supertype and adds attributes or relationships that apply only to that subset. This mirrors inheritance in object-oriented design but is expressed in ER terms. Given Data / Assumptions: Supertype contains shared attributes/keys. Subtypes represent special categories with additional properties or constraints. Constraints: disjoint vs. overlapping; total vs. partial participation. Concept / Approach: Choosing supertype/subtype structures enhances clarity and avoids null-heavy tables. Examples: Person as supertype with subtypes Employee and Student; Account with subtypes Savings and Checking. Each subtype is indeed a “special case” of the supertype. Step-by-Step Solution: Identify common attributes to place in supertype. Define specialized attributes/relationships in subtypes. Select constraints (disjoint/overlap, total/partial) per business rules. Map to relational schema (single-table, class-table, or shared PK strategies). Verification / Alternative check: Validate that each subtype instance is also a valid supertype instance (shared identifier), confirming “is-a” semantics. Why Other Options Are Wrong: Limiting truth to disjoint or total participation mixes orthogonal constraints with the core definition that subtype is a special case of its supertype. Common Pitfalls: Overusing subtypes instead of using simple attributes; forgetting to enforce disjointness/totality rules during implementation. Final Answer: Correct

Question 19

Does “entity class” mean a collection (set) of entity instances characterized by the same structure (attributes and identifier) defined for that class?

Accepted Answer

Introduction / Context: An entity class (or entity type) defines the attributes and identifier that all its instances share. Thinking of “class” as a set of similarly structured instances helps bridge conceptual models to logical schemas and later to tables and rows. Given Data / Assumptions: Entity class/type provides a template (name, attributes, identifier). Instances conform to the template and differ by values. Relationships connect instances across classes according to business rules. Concept / Approach: Treat the entity class as the schema-level definition and each entity as a data-level occurrence. This allows consistent constraints, domains, and keys across all instances and informs table design and normalization decisions. Step-by-Step Solution: Define the entity class (e.g., Product with ProductID, Name, Price). Assert that every Product instance has those attributes and respects domains. Use the identifier to ensure uniqueness across the set of instances. Relate the class to others via relationships (e.g., Product–Supplier). Verification / Alternative check: Ensure that constraints (NOT NULL, CHECK, UNIQUE) apply uniformly across all instances, consistent with the class definition. Why Other Options Are Wrong: Restricting the concept to weak entities or OO models is unnecessary; “entity class” is a general ER term. Common Pitfalls: Blurring instance-level facts with class-level definitions; inconsistent attribute usage across instances. Final Answer: Correct

Question 20

In ER modeling, is “minimum cardinality” the minimum number of entity instances that must (or may) participate in one occurrence of a relationship, thereby expressing optional vs. mandatory participation?

Accepted Answer

Introduction / Context: Minimum cardinality expresses optionality: is participation required (minimum 1) or optional (minimum 0)? Pairing minimum with maximum cardinality yields precise business rules for how entities must or may relate in a given relationship. Given Data / Assumptions: We distinguish minimum (lower bound) from maximum (upper bound) cardinality. Minimum values are typically 0 or 1 in ER practice. Each participating side in a relationship can have its own min/max pair. Concept / Approach: Minimum cardinality tells whether an entity instance must be related at least once to the other side (mandatory) or may be related zero times (optional). For example, an Order must relate to exactly one Customer (min 1, max 1), while a Customer may have zero or many Orders (min 0, max N). Step-by-Step Solution: Identify the relationship and sides (e.g., Customer–Order). Set minimums based on rules: Customer min 0 (optional), Order min 1 (mandatory to a Customer). Set maximums independently (Customer max N; Order max 1). Document these to guide foreign key constraints and application logic. Verification / Alternative check: Review exception cases (e.g., draft orders). If drafts exist without customers, minimum on Order could be 0 until confirmation—showing minimum cardinality encodes real business semantics. Why Other Options Are Wrong: “Incorrect” ignores standard definitions; dependency on diagram style is superficial; limiting to N:M is false since minimum applies to any relationship. Common Pitfalls: Confusing minimum with maximum; forgetting minimums are per side and can differ. Final Answer: Correct

Data Modeling with ER Model Questions