chapter five

5 Relationships

 

In this chapter

  • You establish relationships between entities.
  • You identify the cardinality of each relationship.
  • You make decisions on whether to represent some entities as weak entities.
What you need to know

You can find the database design covered in this chapter (so far it’s entities plus relationships) implemented in tools commonly used by practitioners, such as dbdiagram.io, and MySQL Workbench in the GitHub repository (https://github.com/Neo-Hao/grokking-relational-database-design). You can navigate to the chapter_05 folder, and follow the instructions in README.md to load the database design into corresponding tools.

The data types that show up in this chapter are applicable to most RDBMS, such as MySQL, MariaDB, and PostgreSQL. If you are using another RDBMS such as SQL Server or SQLite, you may need to make small modifications to the design by replacing certain data types with their equivalences specific to an RDBMS. Such information can be found in README.md in the chapter_05 folder.

5.1 Overview

In this chapter, you will develop your entity-relationship (E-R) diagram for The Sci-Fi Collective online store by establishing relationships among entities you identified in Chapter 4. By doing so, you will learn about important concepts in database design, such as cardinality and dependency.

5.2 Entity-relationship models

5.3 Connecting related entities

5.4 Cardinality

5.4.1 Direction and representation

5.4.2 One-to-one: A perfect match

5.4.3 One-to-many: A love triangle

5.4.4 Many to many: The more the merrier

5.5 Strong and weak entities

5.5.1 Identify strong and weak entities in your database

5.6 Summary