cs345-1

cs345-1 - 1 Datalog Rules Programs Negation 2 Review of...

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Unformatted text preview: 1 Datalog Rules Programs Negation 2 Review of Logical If-Then Rules h(X,…) :- a(Y,…) & b(Z,…) & … head body subgoals “The head is true if all the subgoals are true.” 3 Terminology ± Head and subgoals are atoms. ± An atom consists of a predicate (lower case) applied to zero or more arguments(upper case letters or constants). 4 Semantics ± Predicates represent relations. ± An atom is true for given values of its variables iff the arguments form a tuple of the relation. ± Whenever an assignment of values to all variables makes all subgoals true, the rule asserts that the resulting head is also true. 5 Example ± We shall develop rules that describe what is necessary to “make” a file. ± The predicates/relations: ² source(F) = F is a “source” file. ² includes(F,G) = F #includes G. ² create(F,P,G) = F is created by applying process P to file G. 6 Example --- Continued ± Rules to define “view” req(X,Y) = file Y is required to create file X : req(F,F) :- source(F) req(F,G) :- includes(F,G) req(F,G) :- create(F,P,G) req(F,G) :- req(F,H) & req(H,G) Gis required for Fif there is some process P that creates F from G. Gis required for Fif there is some Hsuch that His required for Fand Gis required for H. 7 Why Not Just Use SQL? 1. Recursion is much easier to express in Datalog. ± Viz. last rule for req . 2. Rules express things that go on in both FROM and WHERE clauses, and let us state some general principles (e.g., containment of rules) that are almost impossible to state correctly in SQL. 8 IDB/EDB ± A predicate representing a stored relation is called EDB (extensional database). ± A predicate representing a “view,” i.e., a defined relation that does not exist in the database is called IDB (intesional database). ± Head is always IDB; subgoals may be IDB or EDB. 9 Datalog Programs ± A collection of rules is a (Datalog) program. ± Each program has a distinguished IDB predicate that represents the result of the program. ² E.g., req in our example. 10 Extensions 1. Negated subgoals. 2. Constants as arguments. 3. Arithmetic subgoals. 11 Negated Subgoals ± NOT in front of a subgoal means that an assignment of values to variables must make it false in order for the body to be true....
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cs345-1 - 1 Datalog Rules Programs Negation 2 Review of...

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