CSC 742 Database Management Systems Phases ... - Dr. Peng Ning
CSC 742 Database Management Systems Phases ... - Dr. Peng Ning
CSC 742 Database Management Systems Phases ... - Dr. Peng Ning
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Null Values<br />
Need<br />
Meanings<br />
not applicable<br />
unknown: missing or questionable<br />
existence<br />
Spring 2002 <strong>CSC</strong> <strong>742</strong>: DBMS by <strong>Dr</strong>. <strong>Peng</strong> <strong>Ning</strong> 7<br />
Entities<br />
Entity type (intension): e.g., Employee or<br />
Dept<br />
A collection of entities that have the same<br />
attributes<br />
Entity instance: e.g., Fred or Payroll<br />
Entity set (extension): e.g., {Fred, Bob, ...}<br />
Spring 2002 <strong>CSC</strong> <strong>742</strong>: DBMS by <strong>Dr</strong>. <strong>Peng</strong> <strong>Ning</strong> 8<br />
4
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