Joe Celko's SQL for Smarties, 3rd Edition

2.5. Elementary Key Normal Form (EKNF)

Elementary Key Normal Form (EKNF) is a subtle enhancement on 3NF. By definition, EKNF tables are also in 3NF. This happens when there is more than one unique composite key and they overlap. Such cases can cause redundant information in the overlapping column(s). For example, in the following table, let’s assume that a subject title is also a unique identifier for a given subject in the following table:

CREATE TABLE Enrollment (student_id INTEGER NOT NULL, course_code CHAR(6) NOT NULL, course_name VARCHAR(15) NOT NULL, PRIMARY KEY (student_id, course_name) -- , UNIQUE (student_id, course_name) alternative key ); Enrollment student_id course_name course_name ======================================= 1 'CS-100' ...