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In a relational data model, a relation R has degree 4 and cardinality 10, and a selection operation is applied on R to obtain a relation S with cardinality 5. What is the degree of S?
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4
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5
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10
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20
Answer:
Answer by student
a. 4
Detailed answer by teachoo
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The question is about the relational data model, which represents the database as a collection of relations (or tables). Each relation has a degree and a cardinality. The degree is the number of attributes (or columns) in the relation, and the cardinality is the number of tuples (or rows) in the relation.
Let’s look at each of the options and see why they are correct or incorrect.
- a. 4 : This is the correct option, as the degree of S is the same as the degree of R, which is 4. The selection operation does not change the number of attributes in the relation, only the number of tuples that satisfy a certain condition.
- b. 5 : This is incorrect , as it confuses the degree with the cardinality. The cardinality of S is 5, but the degree is still 4.
- c. 10 : This is incorrect , as it confuses the degree with the cardinality of R. The cardinality of R is 10, but the degree is 4.
- d. 20 : This is incorrect , as it has no relation to the given data.
To get the correct answer, we need to understand that:
- The degree of a relation is determined by its schema , which defines the name and type of each attribute. The degree does not depend on how many tuples are in the relation.
- The cardinality of a relation is determined by how many tuples are in the relation. The cardinality can change depending on what operations are applied to the relation.
- The selection operation selects a subset of tuples from a relation that satisfies a given condition. The selection operation does not alter or add any attributes to the relation, so it does not affect its degree.
So, the correct answer is a. 4
