Second normal form

Second normal form (2NF) is a level of database normalization defined by English computer scientist Edgar F. Codd. A relation (or a table, in SQL) is in 2NF if it is in first normal form (1NF) and contains no partial dependencies. A partial dependency occurs when a non-prime attribute (that is, one not part of any candidate key) is functionally dependent on only a proper subset of the attributes making up a candidate key. To be in 2NF, a relation must have every non-prime attribute depend on the whole set of attributes of every candidate key.

For instance, a relation with the composite key {Country, District} would violate 2NF if any attribute was added whose values' meanings didn't depend on both the Country and the District to which they applied. A CountryLeader attribute would vary between and provide information specific to each Country but not specific to each District, and would therefore depend on only half of the composite key. This would have several drawbacks, including that any leader would be redundantly duplicated for each District in their Country.

The purpose of normalization to 2NF is to reduce such redundancy and to make a database's structure generally more clear and flexible by organizing it by functional dependencies. 2NF and third normal form (3NF) were both defined in Codd's paper "Further Normalization of the Data Base Relational Model" in 1971, a year after Codd defined 1NF in "A Relational Model of Data for Large Shared Data Banks" in 1970. All normal forms make up part of Codd's relational model of database design.