Chapter10(pdf) - KFUPM Open Courseware
Chapter10(pdf) - KFUPM Open Courseware
Chapter10(pdf) - KFUPM Open Courseware
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Null Values in Tuples<br />
• GUIDELINE 3:<br />
• Relations should be designed such that their tuples will<br />
have as few NULL values as possible<br />
• Attributes t that t are NULL frequently could be placed in<br />
separate relations (with the primary key)<br />
• Nulls are problematic in joins and aggregate g functions<br />
• Many interpretations for nulls:<br />
• Attribute not applicable or invalid<br />
• Attribute value unknown (may exist)<br />
• Value known to exist, but unavailable<br />
Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe Slide 10- 15
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