Week 2 (Chapter 4 - Entity Relationship Modelling) Flashcards

1
Q

What is an entity? How is it represented in an ERD?

A

This is an object of interest to the end user. Refers to the entity set and not to a single entity occurency.

Represented by a single rectangle with the entity’s name, name is in all caps AND SINGULAR ONLY

E.g. A table is an entity, a row is not an entity.

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2
Q

What is an entity occurence/instance?

A

This is a single aspect of an entity.

E.g. An entity occurence of a table is a row.

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3
Q

What are attributes?

A

These are the characteristics that describe an entity.

E.g. An entity, student, has attributes such as first/last name, initials, email, phone number etc.

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4
Q

What is the difference between required and optional attributes?

A

A required attribute MUST have a value (cannot be empty)

An optional attribute CAN have empty values.

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5
Q

What is the difference between required and optional attributes?

A

A required attribute MUST have a value (cannot be empty)

An optional attribute CAN have empty values.

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6
Q

What are domains?

A

These are the set of possible values of a given attribute.

E.g. height range, GPA, gender etc.

NOTE: Attributes can share domains.

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7
Q

What are identifiers?

A

These are attributes that uniquely identify each entity instance.

E.g. A primary key is an identifier.

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8
Q

What is the difference between composite and simple attributes?

A

Composite attributes is an attribute that can be subdivided to yield additional attributes

Simple attributes cannot be subdivided.

E.g. An address is a composite attribute (post code, street name, street number, city, state). However, sex or first name are simple attributes.

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9
Q

What is the difference between a single-valued and a multi-valued attribute?

A

Single-valued attributes can only have ONE value.

A multi-valued attribute can have multiple values.

A product’s serial number can only be one value therefore it is single-valued. A multi-valued attribute can incude car colours, heights, material type etc.

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10
Q

What are derived attributes?

A

These are attributes that have a value calculated from other attributes. Not stored, rather calculated with an algorithm.

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11
Q

What are derived attributes?

A

These are attributes that have a value calculated from other attributes. Not stored, rather calculated with an algorithm.

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12
Q

Steps of data base design?

A
  1. Conceptual
  2. Logical
  3. Physical
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13
Q

Can an entity ahve multiple keys?

A

NO, only a single key per entity

can have composite which is a single key created from 2 or more attributes

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14
Q

Are linking tables considered entities?

A

NO

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15
Q

Do you specify primary/foreign keys during the conceptual design stage?

A

No, we DO NOT assume the database type in this stage

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16
Q

What makes a strong entity?

A

An entity that has a key which does NOT need reference to other entities

i.e. has its own unique primary key

17
Q

What makes a weak entity?

A

If it has a key that requires the existence of one or more other entities

i.e. An entity that requires a composite key for identification

18
Q

Can surrogate keys be added during conceptual modelling?

A

NO

19
Q

How do you show that an relationship is identifying?

A

Line between entities is solid.

(one entity points to a weak entity).

20
Q

How do you show that a relationship is non-identifying?

A

Line between entities is dotted.

BOTH strong entities

21
Q

What are derived attributes?

A

These are attributes that can be calculated (using an algorithm)

22
Q

When can you add entities during conceptual modelling?

A

When trying to resolve multi-valued attributes can be shown as a one-to-many relationship. between the parent (original entity) and the child (the multi-variable attribute)

23
Q

What type of entity is a linking/junction table (shows many-to-many relationships)

A

Associative or Composite or Bridging Entity.