When should you use a many-to-many relationship?

A many-to-many relationship is appropriate in scenarios where two entities can have multiple associations with each other. This means that for any given instance of one entity, there can be many related instances of another entity, and vice versa.

In the context of the provided question, the situation where clients have many account managers and account managers have many clients exemplifies a classic many-to-many relationship. Each client can work with multiple account managers, perhaps depending on different aspects of their account or needs, while each account manager can manage multiple clients simultaneously. This interdependent nature between the entities necessitates the use of a many-to-many relationship to effectively model the data.

Considering the other scenarios, they do not exhibit the same kind of bidirectional relationship:

• Project managers with many projects and projects with many tasks describes a one-to-many relationship. A project manager can manage multiple projects, but each project will typically have multiple tasks under it, and the tasks do not relate back to the project managers.

• Contracts having many documents and documents being associated with one contract is also a one-to-many relationship, where each contract can have multiple related documents, but each document corresponds to a single contract.

• The example of account managers having many customers while customers have many addresses is