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NULL Values
Null means no information (no value or an unknown value)
- This leads to a three-valued logic: true/false/unknown
- Conversion of three-valued to two-valued logic:
- when the “where” clause is unknown it is treated as false (meaning the tuple is not selected)
- in an aggregation function (max, avg, count...) unknown values are ignored (tuples are not included in the group)
- in count(*) unknown values are not ignored
- = NULL is always unknown, IS NULL is true or false
Notes: