Apéndice A. Diagrama ER de la base de datos de ejemplo
La Figura A-1 es un diagrama entidad-relación (ER) de la base de datos de ejemplo utilizada en este libro. Como su nombre indica, el diagrama representa las entidades, o tablas, de la base de datos junto con las relaciones de clave externa entre las tablas. Aquí tienes algunos consejos que te ayudarán a entender la notación:
Cada rectángulo representa una tabla, con el nombre de la tabla sobre la esquina superior izquierda del rectángulo. La(s) columna(s) clave principal(es) aparece(n) en primer lugar, seguida(s) de las columnas no clave.
Las líneas entre tablas representan relaciones de clave externa. Las marcas en los extremos de las líneas representan la cantidad permitida, que puede ser cero (0), uno (1) o muchos (<). Por ejemplo, si observas la relación entre las tablas customer y rental, dirías que un alquiler está asociado exactamente a un cliente, pero un cliente puede tener cero, uno o muchos alquileres.
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