Project ForgeDatabase Management Systems
Relational Model & ER Basics
GATE 2017
Medium
An ER model of a database consists of entity types A and B. These are connected by a relationship R which does not have its own attribute. Under which one of the following conditions can the relational table for R be merged with that of A?
- ARelationship R is one-to-many and the participation of A in R is total.
- BRelationship R is one-to-many and the participation of A in R is partial.
- Relationship R is many-to-one and the participation of A in R is total.
- DRelationship R is many-to-one and the participation of A in R is partial.
Explanation
R's table can be absorbed into A's table only if every A-row needs exactly one R-tuple with no nulls — that requires A to be on the 'many' side (many-to-one from A, i.e. each A relates to at most one B) AND total participation of A in R (every A-row does participate, so the merged foreign-key column is never null).
Source: GATE CS 2017 (Session 2) (2017)
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