Database Reference
In-Depth Information
Alias(es)
This is a name (or set of names) that you use for the field in very rare circumstances.
One instance in which you would use an alias is when there must be two occurrences of
the field in the same table. Let's assume that an organization is accustomed to identifying
its employees by unique values within an E MPLOYEE ID N UMBER field. Now, consider the
SUBSIDIARIES table structure in Figure 9.2 (this is a partial structure only).
Figure 9.2. A table requiring two occurrences of the same field
In this instance, each subsidiary has a president and a vice president. Both of these indi-
viduals must be represented in the table because of their positions within the subsidiary
organization, so there are two E MPLOYEE ID N UMBER fields in the table structure. Proper
databasedesign,however,dictatesthattherecanonlybe one occurrenceofthisfieldwithin
the table; there is an obvious problem here. The only solution is to use an alias for one or
both occurrences of the E MPLOYEE ID N UMBER field. For instance, you could (for sake of
clarity) use P RESIDENT ID as an alias for the first occurrence of E MPLOYEE ID N UMBER and
V ICE P RESIDENT ID as an alias for the second occurrence of E MPLOYEE ID N UMBER . With
the aliases in place, both employees are properly represented within the table. Figure 9.3
shows the revised table structure.
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