Null Values Field values in a tuple are sometimes unknown (e.g., a rating has not been assigned) or inapplicable (e.g., no spouse's name).
{"Image DocumentName": "Null ValuesField values in a tuple are sometimes unknown (e.g., a rating has not been assigned) or inapplicable (e.g., no spouse\u2019s name).", "Image ImageDescription": "SQL provides a special value null for such situations. The presence of null complicates many issues. E.g.: Special operators needed to check if value is/is not null. Is rating>8 true or false when rating is equal to null? What about AND, OR and NOT connectives? We need a 3-valued logic (true, false and unknown). Meaning of constructs must be defined carefully. (e.g., WHERE clause eliminates rows that don\u2019t evaluate to true.) New operators (in particular, outer joins) possible/needed.", "Image XResolution": "37", "Image YResolution": "37", "Image ResolutionUnit": "Pixels/Centimeter", "Image Software": "SlidePlayer", "Image YCbCrPositioning": "Centered"}