Figure 4-3: Functional dependency and the determinant. A
Figure 4-3: Functional dependency and the determinant. A determinant is the inversion or opposite of functional dependency. . Transitive dependence Z is transitively dependent on X when X determines Y and Y determines Z. Transitive dependence thus describes that Z is indirectly dependent on X through its relationship with Y. In Figure 4-3, the foreign exchange rates in the RATE field (against the US Dollar) are dependent on CURRENCY. The currency in turn is dependent on COUNTRY. Thus, the rate is dependent on the currency, which is in turn dependent on the country; therefore, RATE is transitively dependent on COUNTRY. . Candidate key A candidate key (potential or permissible key) is a field or combination of fields that can act as a primary key field for a table thus uniquely identifying each record in the table. Figure 4-4 shows five different variations of one table, all of which have valid primary keys, both of one field and more than one field. The number of options displayed in Figure 4-4 is a little ridiculous, but demonstrates the concept. FXCODE ALL BGN CYP CZK DKK DM HUF ISK MTL NOK PLN ROL SEK CHE GBP CURRENCY Leke Leva Pounds Koruny Krener Deutsche Marks Forint Kronur Liri Krone Zlotych Lei Kronor Francs Pounds COUNTRY Albania Public Denmark Germany Hungary Iceland Malta Norway Poland Romania RATE 5.8157 1.5 6.5412 7.000 10.00 0.538 RATE DM determines that the currency is Deutsche Marks Pounds is dependant on the code being GBP 77 Understanding Normalization
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