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Entity-Relationship Model. Relationships
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Relational Database and Normalization

Entity-Relationship Model. RelationshipsEntity-Relationship Model. Relationships

Relationships in Databases

In relational databases, various relationships can be illustrated in diagrams.

Different entities can have specific interdependencies, each defining how elements interact. An element might relate to multiple others or just one. This interaction is termed a relationship.

Relational databases capture these connections in diagrams.

Types of Relationships:

  • One-to-One
  • One-to-Many
  • Many-to-One
  • Many-to-Many

We'll dive deeper into these relationship types later, but for now, here's their representation:

Mandatory vs. Optional Relationships

Mandatory and optional relationships come with distinct markers: | (a line) for mandatory and o (a circle) for optional.

  • A mandatory relationship implies that a particular entity instance must have a connection to another entity.
  • An optional relationship means that an entity instance might have a link to another entity, but it's not obligatory.

Consider the following examples:

While mandatory and optional relationships might not be frequently used, it's crucial to recognize these markers.

1. Select all relationship types:
2. Choose marks:


Select all relationship types:

Select a few correct answers


Choose marks:

The mandatory mark is .

The optional mark is

Click or drag`n`drop items and fill in the blanks

Everything was clear?

Section 1. Chapter 4