The size of the data sets and the available system resources, ensuring the join operation does not exceed the allocated budget and complies with data handling policies.

The relationships between the data sets and the desired output, focusing on maintaining data integrity and meeting the analysis objectives without unnecessary data duplication.

The performance requirements of the project and the expected query response time, to ensure the solution meets the SLA for data availability and processing speed.

All of the above, as each factor plays a vital role in selecting the most appropriate join type that aligns with the project's cost, compliance, and scalability requirements.