Third normal form

Common card · No. 256642

Third normal form (3NF) is a level of database normalization defined by English computer scientist Edgar F. Codd. A relation (or table, in SQL) is in third normal form if it is in second normal form and also lacks non-key dependencies, meaning that no non-prime attribute is functionally dependent on (that is, contains a fact about) any other non-prime attribute. In other words, each non-prime attribute must depend solely and non-transitively on each candidate key. William Kent summarised 3NF with the dictum that "a non-key field must provide a fact about the key, the whole key, and nothing but the key". An example of a violation of 3NF would be a Patient relation with the attributes PatientID, DoctorID and DoctorName, in which DoctorName would depend first and foremost on DoctorID and only transitively on the key, PatientID (via DoctorID's dependency on PatientID). Such a design would cause a doctor's name to be redundantly duplicated across each of their patients. A database compliant...

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