Question
To be used as a primary key, which of these characteristics must an attribute have in a single instance of the entity? Select all that apply. There must be a maximum of one attribute instance. There must be many instances of the attribute. There must be at most one entity instance that each attribute instance describes. There must be at least one instance of the attribute. There must be zero or more instances of the attribute.
Ask by Ford Washington. in the United States
Feb 02,2025
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Answer
To be used as a primary key, an attribute must:
- Have a maximum of one instance.
- Uniquely identify one entity instance.
- Have at least one instance.
Therefore, the correct options are:
- There must be a maximum of one attribute instance.
- There must be at most one entity instance that each attribute instance describes.
- There must be at least one instance of the attribute.
Solution
To determine the characteristics that an attribute must have to be used as a primary key in a single instance of an entity, let's analyze each option:
1. **There must be a maximum of one attribute instance.**
- This is true for a primary key. A primary key must uniquely identify a single instance of an entity, meaning there cannot be multiple instances of the primary key for the same entity.
2. **There must be many instances of the attribute.**
- This is not true. A primary key should uniquely identify each instance of an entity, so it should not have many instances. Instead, it should have exactly one instance per entity.
3. **There must be at most one entity instance that each attribute instance describes.**
- This is true. A primary key must uniquely identify one entity instance, meaning that each value of the primary key can only correspond to one instance of the entity.
4. **There must be at least one instance of the attribute.**
- This is true. For an attribute to serve as a primary key, it must have at least one instance to identify an entity.
5. **There must be zero or more instances of the attribute.**
- This is not true. A primary key cannot have zero instances; it must have at least one instance to identify an entity.
### Summary of Correct Characteristics:
- There must be a maximum of one attribute instance.
- There must be at most one entity instance that each attribute instance describes.
- There must be at least one instance of the attribute.
Thus, the correct characteristics for an attribute to be used as a primary key are:
- There must be a maximum of one attribute instance.
- There must be at most one entity instance that each attribute instance describes.
- There must be at least one instance of the attribute.
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Simplify this solution Beyond the Answer
When selecting a primary key, it’s essential to remember that each attribute must uniquely identify a specific entity instance, ensuring no two instances share the same key value. This means there can only be one instance of a primary key attribute for each entity, like a social security number for each person, thus making that individual distinct in the database. Furthermore, the primary key attribute cannot have multiple values for any given entity instance. This principle keeps your database from going haywire and ensures consistent and reliable data referencing. Think of it like having a unique license plate for each car; it prevents confusion and keeps the system organized!
