Context

You are comparing customer records from two systems, such as a CRM and a billing table, where some customers exist in only one system. The join choice determines whether you keep only the left-side population or preserve unmatched rows from both sides.

Core question

Explain the difference between a LEFT JOIN and a FULL OUTER JOIN in a customer use case. Describe what rows each join returns, how NULL values appear, and when each join is the better choice if you need to identify matched customers, missing billing records, or customers that exist in only one source.

Scope guidance

Keep your answer practical and focused on result shape and business interpretation. You should be able to explain the join behavior clearly enough that a non-technical stakeholder would understand why one join would hide certain customers while the other would surface them.