Relationship between Tables

Learn about SQL table relationships and how queries are carried out using TypeORM and NestJS for the API layer.

By table relationships, we mean the relationships between various tables or entities as they are described in TypeORM.

Just to review, here are the three basic types of table relationships:

  • One-to-one
  • One-to-many
  • Many-to-many

In this lesson, we’ll look in-depth at the last two and their associated queries.


A one-to-one relationship is one in which an entity uniquely maps to another. A good example is a relationship that exists between a person and their fingerprint.

In TypeORM, we can achieve this using the @OneToOne decorator. Below, we define two tables with a few properties for both FingerPrint and Person classes. We use the @OneToOne decorator to specify the relationship between them.

We’re also using the @JoinColumn decorator. This specifies which table we want to include the foreign key. Keep in mind that only one of the tables should include the foreign key.

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