The Relational Database Dictionary, Extended Edition

Chapter 11. J

JD Join dependency.

JD implied by superkeys The JD

Example: Let relvar R have attributes A, B, C, D, E, and F (only), and let it have keys {A}, {B}, and {C} (only). Let ABD denote the set of attributes {A,B,D}, and similarly for other letter combinations. Then the first of the following JDs is implied by the superkeys of R but the second and third aren't:

 { ABD, BCE, CF }
 { ABD, CE, CF }
 { ABD, BCE, DF }

Note that relvar R necessarily ...