- For clarity, the database instance should ideally accord with
all and only those constraints which are explicitly stated in
the definition of the database schema.
- For simplicity and ease of typing, attribute values should be
short and salary values should be multiples of $10,000.
- Transitions (i.e., timestamp values) occur only at the
beginning of the month, and all dates should be in the time interval
from 1/1/81 to 12/31/88 (because the digits 8 and 9 are relatively
hard to distinguish). Time intervals are all specified by the
inclusive starting and ending events. Also for clarity, relation
instance names should start with lowercase letters.
- The data should include a ``hole in the history'' of some
entity. For example, the database may be designed to contain a whole
in the employment of some employee.
- The data should include asynchronous behavior of attributes.
For example, the department and salary of employees may change
independently.