Ambiguous Oracle by Spider Robinson

By Spider Robinson

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Each passenger must have a seat, and only one, so card(Passengers, seat_assign) = (1, 1), and seats can be used by at most one passenger and may go empty, so card(Seats, seat_assign) = (0, 1). 14. Now the ER design is complete, and we need to transform the design into relational tables. We can begin by creating tables to map entities, even though this means that we might overlook some attributes that will be needed to represent foreign keys for relationships. We will simply have to return later when we consider the relationships and add attributes to these tables.

If the Orders occurrence goes away (the customer cancels it), all occurrences of the weak entity Line_items will likewise disappear. A dead giveaway for a weak entity is the fact that the primary identifier for Line_items (lineno) is only meaningful within some order. 11 A weak entity, Line_items, dependent on the entity Orders. 3 Additional ER Concepts 31 Orders entity. Attributes such as Line_items are known as external identifier attributes. When the Line_items weak entity is mapped to a relational table line_items, an ordno column is included by Transformation Rule 4 to represent the N-1 has_item relationship; thus, the primary key for the line_items table is constructed from the external attribute ordno and the weak entity identifier lineno.

On the other hand, if more than one instructor were allowed to share the teaching of a course section, the DBA would write max-card(Course_sections, teaches) = N. This is clearly a significant difference. We probably don’t want to make the rule that every instructor teaches some course section (written as min-card(Instructors, teaches) = 1), because an instructor might be on leave, so we settle on min-card(Instructors, teaches) = 0. And in most universities the course load per instructor is greater than one in any given term, so we would set max-card(Instructors, teaches) = N.

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