Figure 4-4: A table with five and possibly (Adult web hosting)
Tuesday, May 8th, 2007Figure 4-4: A table with five and possibly more candidate keys. . Full functional dependence This situation occurs where X determines Y, but X combined with Z does not determine Y. In other words, Y depends on X and X alone. If Y depends on X with anything else, there is not full functional dependence. Essentially X, the determinant, cannot be a composite key. A composite key contains more than one field (the equivalent of X with Z). Figure 4-5 shows that POPULATION is dependent on COUNTRY but not on the combination of RATE and COUNTRY. Therefore, there is full functional dependency between POPULATION and COUNTRY because RATE is irrelevant to POPULATION. Conversely, there is not full functional dependence between POPULATION and the combination of COUNTRY and RATE. Customer customer_id customer currency_code currency exchange_rate address Customer currency_code customer customer_id currency exchange_rate address Customer currency customer_id address customer currency_code exchange_rate Customer address customer_id customer currency_code currency exchange_rate Customer customer customer_id currency_code currency exchange_rate address Customer customer_id customer currency_code currency exchange_rate address 78 Chapter 4
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