Join Relationships

Join relationships define how tables connect. A relationship specifies: which tables join (e.g., orders and customers), on which keys (customer_id = orders.customer_id), what cardinality exists (one customer has many orders), and what join type applies (inner, left outer, etc.). Rather than leaving join logic implicit in SQL, a semantic model makes these relationships explicit and reusable.

Join relationships solve fragility and inconsistency in analytics. Without formalized relationships, different analysts write different JOINs: some use left outer join, others inner; some join on customer_id, others on email address. This produces different results. Relationship definitions enforce consistency: the semantic layer ensures all queries joining orders to customers use the same logic. Relationships also prevent join errors like unintended many-to-many multiplications that inflate metrics.

Relationships are typically bidirectional: knowing orders-to-customers relationships enables reverse joins (find customers without orders). Some relationships are conditional: a join only applies when certain filters are present. The semantic model documents relationship strength: whether a join is 1:1, 1:many, many:1, or many:many, which determines correct aggregation. Invalid joins (crossing incompatible cardinalities) can be flagged automatically.