What Are Relationships?

Why Cardinality Matters More Than You Think When you look at a schema, the first thing to understand is not the column names. It is the cardinality of the relationships between tables. Cardinality tells you whether a join will produce the same number of rows, more rows, or fewer rows than the input. Getting this wrong is the single most common source of incorrect query results in production systems. Relationships are implemented using foreign keys, but the foreign key is just the mechanism. The real content is the business rule: can one customer have many orders? Can one order belong to many customers? The answer to these questions determines the shape of every downstream query. The line connecting these tables represents a one-to-many relationship. customer_id in orders is a foreign key r