referential integrity ensures that data is consistent between tables. Specifically, referential constraints ensure that a foreign key value in a child table (or child row in the case of a recursive constraint) always has a matching primary key value in the parent table (or parent row). In a physical design, a foreign key is the combination of one or more columns in a table that reference (match) the primary key in the parent table (or parent row in the case of recursive relationships). Primary keys and foreign keys are the fundamental building blocks of the relational database because these are the components that make the relationships work while allowing tables to be accessed independently. One-to-many relationships between primary keys and foreign keys are thus defined as referential constraints in the physical database.