5 Relationships

 

In this chapter

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

You can find the database design covered in this chapter 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 apply to most relational database management systems (RDBMSs), such as MySQL, MariaDB, and PostgreSQL. If you use another RDBMS, such as SQL Server or SQLite, you may need to make small modifications to the design by replacing certain data types with equivalents specific to the target RDBMS. You can find such information in README.md in the chapter_05 folder.

Overview

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

Entity-relationship diagrams

Connect related entities

Cardinality

Direction and representation

One-to-one: A perfect match

One-to-many: A love triangle

Strong and weak entities