Master-Detail Relationship Explained
Master-detail relationships enforce a strong dependency between parent (master) and child (detail) objects. The detail record cannot exist without a master; the relationship field is always required and cannot be blank. The detail record inherits its OWD from the master, meaning you cannot set independent sharing on the detail object. If the master record is deleted, all detail records are automatically deleted (cascade delete). Ownership of detail records is irrelevant; access is controlled entirely through the master.
The key advantage of master-detail relationships is support for roll-up summary fields on the master object, which can calculate COUNT, SUM, MIN, or MAX across detail records, extremely useful for aggregating line items on an order, totaling opportunity amounts on an account, or counting tasks on a project. Each object can participate in up to two master-detail relationships, enabling junction objects for many-to-many modeling. Converting between lookup and master-detail requires that every child record has a populated parent reference. Clientell AI can analyze your data model and identify where converting lookups to master-detail would unlock roll-up summaries and simplify your security model.