Codd's Rules No 03: Nulls are Treated Uniformly As Unknown
- Null must always be interpreted as an unknown value
- 'Unknown' is not the same thing as an empty string ("") or zero
- Comparing a null to any value, including itself, returns NULL
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- Codd's Rule No 12: Data Integrity Cannot be Subverted
- Codd's Rules No 11: Supports Distributed Operations
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- Oracle9i and above: How to reclaim (empty) TEMP ta...
- Codd's Rules No 09: Logical Data Independence
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- Codd's Rules No 04: Database is Self-Describing
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