What this topic covers
12 subtopics- ER to tables13
- Cardinality5
- Design5
- Relationships5
- Attributes4
- Weak entities4
- Generalisation and specialisation3
- Participation2
- Aggregation1
- Entities1
- Min–max notation1
- Notation1
- Beginner 15
- Intermediate 15
- Advanced 15
About this topic
The Entity–Relationship (ER) model describes data at the design stage: entities (things), their attributes, and the relationships between them, with cardinality (1:1, 1:N, M:N) and participation (total or partial). Interviews test the vocabulary and the design judgement; GATE regularly asks how many tables an ER diagram needs. The mapping rules used throughout: a strong entity becomes a table; an M:N relationship becomes its own table; a 1:N relationship becomes a foreign key on the N side; a multivalued attribute becomes a separate table.
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Sample questions
Try each one first, then open the answer for the explanation and why the other options are wrong.
In an ER model, what is the difference between an entity and an entity set?
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Explanation
An entity is a single object with an identity; an entity set (or entity type) groups entities that share the same attributes. It maps to a table, and each entity to a row.
Why the other options are wrong
- AOne is an instance, the other the collection.
- BThis reverses the two.
- CAttributes describe entities.
What is a weak entity?
Show answer and explanation
Explanation
A weak entity is identified by its owner's key plus its own partial key. Example: Dependent (name) of an Employee – "Riya" is unique only within one employee's dependents.
Why the other options are wrong
- AUsage has nothing to do with it.
- CThe number of attributes is irrelevant.
- DA weak entity always has an identifying relationship.