ER diagram notation in DBMS Theory - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does a rectangle represent in an ER diagram?

A rectangle represents an Entity, which is a real-world object or concept that can have data stored about it.

beginner

What symbol is used to represent an attribute in an ER diagram?

An ellipse (oval shape) is used to represent an attribute, which describes properties of an entity or relationship.

beginner

How is a relationship shown in an ER diagram?

A diamond shape represents a relationship, showing how two or more entities are connected.

intermediate

What does a double ellipse indicate in ER diagram notation?

A double ellipse represents a multivalued attribute, meaning the attribute can have multiple values for a single entity.

intermediate

What is the meaning of a double rectangle in an ER diagram?

A double rectangle represents a weak entity, which depends on another entity (called the owner) for its identification.

Which shape is used to represent an entity in an ER diagram?

A Ellipse

B Double ellipse

C Diamond

D Rectangle

Entities are shown as rectangles in ER diagrams.

What does a diamond shape represent in an ER diagram?

A Attribute

B Relationship

C Entity

D Weak entity

A diamond shape represents a relationship between entities.

In ER notation, what does a double ellipse indicate?

A Multivalued attribute

B Derived attribute

C Primary key

D Weak entity

A double ellipse shows a multivalued attribute.

Which symbol represents a weak entity in an ER diagram?

A Diamond

B Ellipse

C Double rectangle

D Single rectangle

Weak entities are shown with double rectangles.

What does an underline under an attribute name signify in ER diagrams?

A Primary key attribute

B Derived attribute

C Foreign key attribute

D Multivalued attribute

An underline indicates the attribute is a primary key.

Explain the main symbols used in ER diagram notation and what each represents.

Think about shapes and what real-world concepts they stand for.

Describe how you would identify a weak entity and a multivalued attribute in an ER diagram.

Look for double shapes.

Practice

1. Which shape in an ER diagram represents an entity?

easy

A. Rectangle

B. Oval

C. Diamond

D. Triangle

Solution

1. Step 1: Recall ER diagram shapes

   Entities are shown as rectangles in ER diagrams.

2. Step 2: Match shape to entity

   Rectangles represent entities, ovals represent attributes, diamonds represent relationships.

3. Final Answer:

   Rectangle -> Option A

4. Quick Check:

   Entity = Rectangle [OK]

Hint: Entities are rectangles, attributes are ovals [OK]

Common Mistakes:

• Confusing attributes (ovals) with entities
• Thinking diamonds are entities
• Mixing shapes with cardinality symbols

2. In ER diagrams, which shape is used to represent an attribute?

easy

A. Diamond

B. Rectangle

C. Hexagon

D. Oval

Solution

1. Step 1: Identify attribute symbol

   Attributes are shown as ovals in ER diagrams.

2. Step 2: Confirm shape usage

   Ovals represent attributes, rectangles represent entities, diamonds represent relationships.

3. Final Answer:

   Oval -> Option D

4. Quick Check:

   Attribute = Oval [OK]

Hint: Attributes are always ovals in ER diagrams [OK]

Common Mistakes:

• Choosing diamond instead of oval
• Confusing entity and attribute shapes
• Selecting shapes not used in ER diagrams

3. Consider an ER diagram where a diamond connects two rectangles with a line labeled '1' on one side and 'N' on the other. What does this cardinality mean?

medium

A. One entity relates to one entity only

B. Many entities relate to one entity

C. One entity relates to many entities

D. Many entities relate to many entities

Solution

1. Step 1: Understand cardinality symbols

   '1' means one entity, 'N' means many entities.

2. Step 2: Interpret relationship

   The side with '1' relates to many entities on the 'N' side, so one entity relates to many entities.

3. Final Answer:

   One entity relates to many entities -> Option C

4. Quick Check:

   Cardinality 1 to N = One to many [OK]

Hint: 1 to N means one entity connects to many [OK]

Common Mistakes:

• Reversing the direction of cardinality
• Confusing 'N' with one
• Ignoring the diamond relationship

4. Identify the error in this ER diagram description: "A diamond represents an attribute, and an oval represents a relationship."

medium

A. Diamond represents a relationship, not an attribute

B. Oval represents an entity, not a relationship

C. Rectangle represents an attribute, not a relationship

D. There is no error in the description

Solution

1. Step 1: Recall ER diagram shape meanings

   Diamonds represent relationships, ovals represent attributes.

2. Step 2: Check the description correctness

   The description wrongly says diamond is attribute and oval is relationship, which is incorrect.

3. Final Answer:

   Diamond represents a relationship, not an attribute -> Option A

4. Quick Check:

   Diamond = Relationship [OK]

Hint: Diamonds = relationships, ovals = attributes [OK]

Common Mistakes:

• Mixing diamond and oval meanings
• Thinking rectangles are attributes
• Assuming no error exists

5. You want to model a "Student" entity with attributes "StudentID" (unique), "Name", and "PhoneNumber" in an ER diagram. Which notation correctly shows the uniqueness of "StudentID"?

hard

A. "StudentID" attribute inside a diamond connected to the Student rectangle

B. "StudentID" attribute underlined inside an oval connected to the Student rectangle

C. "StudentID" attribute inside a rectangle connected to the Student oval

D. "StudentID" attribute inside an oval with a double border connected to the Student rectangle

Solution

1. Step 1: Recall notation for key attributes

   Key (unique) attributes are shown as underlined text inside ovals.

2. Step 2: Apply notation to "StudentID"

   "StudentID" should be underlined inside an oval connected to the Student entity rectangle.

3. Final Answer:

   "StudentID" attribute underlined inside an oval connected to the Student rectangle -> Option B

4. Quick Check:

   Key attribute = Underlined oval [OK]

Hint: Underline key attributes inside ovals [OK]

Common Mistakes:

• Using diamond for attributes
• Placing attributes inside rectangles
• Using double border for uniqueness