Losa Chapter 3

After learning the 3rd chapter, can you explain to me:

1. What are entity, entity type, and entity instance?

Entity – a person, place, object, event, or concept in the user environment about which the organization wishes to maintain data.

Entity Instance- A single occurrence of an entity type: often corresponds to a row in a table

2. What are strong and weak entities?

Strong entities exist independently of other type of entities. Instances of a strong entity type always have a unique characteristic.

Weak entities Depend on some other entity type, has no business meaning in an E R diagram without the entity on which it depends, and does not have a unique identifier (only a partial identifier).

3. What are required, optional, composite, and simple attributes?

Required must be present for each entity instance.

Optional may not have a value for an entity instance

Composite is an attribute, such as address, that has meaningful component parts.

Simple Attribute is an attribute that cannot be broken down into smaller components.

4. What are single-valued, multi-valued, stored, and derived attributes?

Single-value has only one value

Multivalued may take on more than one value of a given entity instance

Stored Attribute cannot be calculated or derived from other related attribute values

Derives Attribute is an attribute whose values can be calculated or derived from related attribute values

5. What are identifier (Key), composite identifier, and candidate identifier?

Identifier is an attribute that uniquely distinguishes individual instances of an entity type.

Composite Identifier consists of more than one attributes.

Candidate is an attribute that satisfies the requirements for being an identifier.

Key will not change in value and cannot be null.

6. What are relationship and relationship instance?

Relationship is an association representing an interaction among the instances of one or more entity types...