In addition to being represented by their own records, such so-called “attributes” can exhibit another characteristic property of entities (or relationships): they themselves might be the subject of some information (like intersection data). Thus, the intersection records required to list the children of employees might include their birthdays; the intersection records listing the colors of multi-colored cars might include the percentage of a car’s surface covered by a given color. To summarize, consider the contradictions in the following assertions:

• Every record represents an entity.
• Every entity is represented by a distinct record.
• All relationships are entities.
• Some relationships are not represented by distinct records.
• The “subject” of an attribute is an entity.
• Some attributes are the subjects of other attributes.
• Attributes are not entities.
• Some records represent attributes (and nothing else).

You play the game. See how many contradictory combinations you can find.

Then decide which of those assertions you’re willing to give up in order to achieve consistency.