Explicando la cardinadinalidad de una ternaria (con Hanna Montana)

Ejemplo de ayer por la tarde, en una de mis clases de Ingenieria de Software:


Tenemos esta asociación ternaria entre Artista, Canción e Instrumento. Un artista puede tocar uno o más instrumentos en una canción. Un instrumento sólo puede ser tocado por un artista en cada canción.

Vamos a hablar de las cardinalidades mínimas de la asociación. Empecemos por el extremo instrumento.

Si ponemos una cardinalidad mínima de 1, significaría que, para cada pareja del producto cartesiano de Artistas y Canciones, debería haber, como mínimo, un instrumento.

Eso significa que Hanna Montana estaría obligada a tocar el ukelele en la canción Master of Puppets. Cosa que no debería poder pasar jamás por nuestra propia salud mental. Así pues, la cardinalidad mínima de ese extremo de la asociación es un 0.


Qué ejemplos nos tenemos que inventar los profes, gensanta!

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