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| Both sides previous revision Poprzednia wersja Nowa wersja | Poprzednia wersja | ||
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pl:miw:2009:present:alsvfd [2009/05/24 23:20] jsi08 |
pl:miw:2009:present:alsvfd [2019/06/27 15:50] (aktualna) |
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| ===== ANY and NULL ===== | ===== ANY and NULL ===== | ||
| - | In case the value of A_i is unspecified we shall write A_i = NULL (a database convention). | + | * In case the value of A_i is unspecified we shall write A_i = NULL (a database convention). |
| - | + | * Following a Prolog convention and logic, a //ANY// attribute value is possible in comparison (see''_'' in Prolog). | |
| - | Following a Prolog convention and logic, a //ANY// attribute value is possible in comparison (see''_'' in Prolog). | + | * The semantics can be: "any value", "not important", etc. |
| - | + | * The solution: | |
| - | The semantics can be: "any value", "not important", etc. | + | * in preconditions, we can only use //ANY//, i.e. an atom such as ''A=_'' can be specified, meaning "any value", "all possible values of the attribute", "we don't care" |
| - | + | * on the other hand, attribute A unspecified, in the state formula means ''A=NULL'', so we store NULL in state | |
| - | The solution: | + | * here we come to an inference rule: ''A=NULL'' ==> ''A=_''. Seems to be valid... This rules should be optionally disabled/enabled in the inference engine. |
| - | * in preconditions, we can only use //ANY//, i.e. an atom such as ''A=_'' can be specified, meaning "any value", "all possible values of the attribute", "we don't care" | + | |
| - | * on the other hand, attribute A unspecified, in the state formula means ''A=NULL'', so we store NULL in state | + | |
| - | * here we come to an inference rule: ''A=NULL'' ==> ''A=_''. Seems to be valid... This rules should be optionally disabled/enabled in the inference engine. | + | |
| - | + | ||
| - | FIXME It seems, we could have three types of NULL-like values: Not-applicable, Potentially-applicable but taking no value empty/no-defined, Applicabe-and-takin-value but unknown. | + | |
