Conjunctive queries examples

  • What is a short note on a conjunctive query?

    In database theory, a conjunctive query is a restricted form of first-order queries using the logical conjunction operator.
    Many first-order queries can be written as conjunctive queries.
    In particular, a large part of queries issued on relational databases can be expressed in this way..

  • What is the union of conjunctive queries?

    The class UCQ (Union of Conjunctive Queries) is obtained by adding union to conjunctive queries.
    A UCQ is a query of the form q1 ∪ q2 ∪ qm, where each qi is a conjunctive query..

  • Conjunctive selection using an individual index: If an attribute involved in any single simple condition in the conjunctive condition has an access path that permits the use of one of the methods S2 to S6 to retrieve the records and then check whether each retrieved record satisfies the remaining conditions in the
A UCQ is a query of the form q1 ∪ q2 ∪ qm, where each qi is a conjunctive query. For example, the query q = q1 ∪ q2, where q1(x, y) = R(x, z), R(z, y), and q2(x, y) = R(x, z), R(z, w), R(w, y) is a UCQ that returns the endpoints of paths of length 2 or 3.

Can a first order query be a conjunctive query?

Many first-order queries can be written as conjunctive queries.
In particular, a large part of queries issued on relational databases can be expressed in this way.
Conjunctive queries also have a number of desirable theoretical properties that larger classes of queries (e.g., the relational algebra queries) do not share.

,

How to write a conjunctive query in Datalog form?

When writing CQs in Datalog form, we can also choose to use equality (=).
For example, we can equivalently write the query q(x):

  • R(x
  • a) as q(x) :
  • R(x
  • y)
  • y = a.
    However, we are not allowed to use any other predicate symbols, such as : , 6 =.
    So far we looked at the syntactic definition of a conjunctive query.
    • ,

    Is a conjunctive query acyclic?

    Acyclicity of conjunctive queries is a structural property of queries that is defined with respect to the query's hypergraph:

  • a conjunctive query is acyclic if and only if it has hypertree-width 1.
    • ,

    What are some examples of conjunctive queries?

    Below are some examples of conjunctive queries:

  • When writing CQs in Datalog form
  • we can also choose to use equality (=).
    For example, we can equivalently write the query q(x):R(x, a) as q(x):R(x, y), y = a.
    However, we are not allowed to use any other predicate symbols, such as : , 6 =.
    • In the theory of relational databases, a Boolean conjunctive query is a conjunctive query without distinguished predicates, i.e., a query in the form mwe-math-element>, where each mwe-math-element> is a relation symbol and each mwe-math-element> is a tuple of variables and constants; the number of elements in mwe-math-element> is equal to the arity of mwe-math-element>.
    Such a query evaluates to either true or false depending on whether the relations in the database contain the appropriate tuples of values, i.e. the conjunction is valid according to the facts in the database.
    In database theory and systems, a monotonic query is one that does not lose any tuples it previously made output, with the addition of new tuples in the database.
    Formally, a query q over a schema R is monotonic if and only if for every two instances I, J of R, mwe-math-element> (q must be a monotonic function).

    Categories

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