Results 1 
9 of
9
A comparative study of various nested normal forms
 IEEE Transactions on Knowledge and Data Engineering
, 2002
"... AbstractÐAs objectrelational databases �ORDBs) become popular in the industry, it is important for database designers to produce database schemes with good properties in these new kinds of databases. One distinguishing feature of an ORDB is that its tables may not be in first normal form. Hence, OR ..."
Abstract

Cited by 14 (5 self)
 Add to MetaCart
AbstractÐAs objectrelational databases �ORDBs) become popular in the industry, it is important for database designers to produce database schemes with good properties in these new kinds of databases. One distinguishing feature of an ORDB is that its tables may not be in first normal form. Hence, ORDBs may contain nested relations along with other collection types. To help the design process of an ORDB, several normal forms for nested relations have recently been defined, and some of them are called nested normal forms. In this paper, we investigate four nested normal forms, which are NNF [20], NNF [21], NNF [23], and NNF [25], with respect to generalizing 4NF and BCNF, reducing redundant data values, and design flexibility. Another major contribution of this paper is that we provide an improved algorithm that generates nested relation schemes in NNF [20] from anacyclic database scheme, which is the most general type of acyclic database schemes. After presenting the algorithm for NNF [20], the algorithms of all of the four nested normal forms and the nested database schemes that they generate are compared. We discovered that when the given set of MVDs is not conflictfree, NNF [20] is inferior to the other three nested normal forms in reducing redundant data values. However, in all of the other cases considered in this paper, NNF [20] is at least as good as all of the other three nested normal forms. Index TermsÐObjectrelational database management systems, objectrelational databases, SQL:1999, nested relation schemes, nested relations, nested database schemes, nested databases, nested normal forms, conflictfree sets of MVDs, acyclic database schemes, nested database design, data redundancy, design flexibility, algorithms. 1
Independent database schemes under functional and inclusion dependencies
 Proc. of the 13th Int. Conf. on Very Large Data Bases, VLDB’87
, 1987
"... In a context considering in a unique framework all the relations in a database, by means of the notion of global consistency, independent database schemes allow enforcement of constraints to be performed JocaJJy, thus providing independent updatability of the various relations. Independent schemes ..."
Abstract

Cited by 9 (1 self)
 Add to MetaCart
In a context considering in a unique framework all the relations in a database, by means of the notion of global consistency, independent database schemes allow enforcement of constraints to be performed JocaJJy, thus providing independent updatability of the various relations. Independent schemes have hitherto been studied in the presence of functionirl and join dependencies. In this paper we extend the definition and give some cbaracterisations when the involved set of constraints is composed of functional and inclusion dependencies. 1.
Updating Relational Databases Through Weak Instance Interfaces
 ACM TODS
, 1992
"... The problem of updating databases through interfaces based on the weak instance model is studied, thus extending previous proposals that considered them only from the query point of view. Insertions and deletions of tuples are considered. As a preliminary tool, a lattice on states is defined, based ..."
Abstract

Cited by 8 (1 self)
 Add to MetaCart
(Show Context)
The problem of updating databases through interfaces based on the weak instance model is studied, thus extending previous proposals that considered them only from the query point of view. Insertions and deletions of tuples are considered. As a preliminary tool, a lattice on states is defined, based on the information content of the various states. Potential results of an insertion are states that contain at least the information in the original state and that in the new tuple. Sometimes there is no potential result, and in the other cases there may be many of them. We argue that the insertion is deterministic if the state that contains the information common to all the potential results (the greatest lower bound, in the lattice framework) is a potential result itself. Effective characterizations for the various cases exist. A symmetric approach is followed for deletions, with fewer cases, since there are always potential results; determinism is characterized consequently. 1 Introdu...
Computing full disjunctions
 In PODS
, 2003
"... Under either the orsemantics or the weak semantics, the answer to a query over semistructured data consists of maximal rather than complete matchings, i.e., some query variables may be assigned null values. In the relational model, a similar effect is achieved by computing the full disjunction (rat ..."
Abstract

Cited by 5 (3 self)
 Add to MetaCart
(Show Context)
Under either the orsemantics or the weak semantics, the answer to a query over semistructured data consists of maximal rather than complete matchings, i.e., some query variables may be assigned null values. In the relational model, a similar effect is achieved by computing the full disjunction (rather than the natural join or equijoin) of the given relations. It is shown that under either the orsemantics or the weak semantics, query evaluation has a polynomialtime complexity in the size of the query, the database and the result. It is also shown that the evaluation of full disjunctions is reducible to query evaluation under the weak semantics and hence can be done in polynomial time in the size of the input and the output. Complexity results are also given for two related problems. One is evaluating a projection of the full disjunction and the other is evaluating the set of all tuples in the full disjunction that are nonnull on some given attributes. In the special case of γacyclic relation schemes, both problems have polynomialtime algorithms in the size of the input and the output. In the general case, such algorithms do not exist, assuming that P = NP. Finally, it is shown that the weak semantics can generalize full disjunctions by allowing tuples to be joined according to general types of conditions, rather than just equalities among attributes. 1.
Decomposition of relational schemata into components defined by both projection and restriction
 ACM SIGACTSIGMOSSIGART Sym
, 1988
"... A generalized approach to the decomposition of relational schemata is developed in which the component views may be defined using both restriction and projection operators, thus admitting both horizontal and vertical decompositions. The realization of restrictions is enabled through the use of a Boo ..."
Abstract

Cited by 5 (2 self)
 Add to MetaCart
(Show Context)
A generalized approach to the decomposition of relational schemata is developed in which the component views may be defined using both restriction and projection operators, thus admitting both horizontal and vertical decompositions. The realization of restrictions is enabled through the use of a Boolean algebra of types, while true independence of projections is modelled by permitting null values in the base schema. The flavor of the approach is algebraic, with the the collection of all candidate views of a decomposition modelled within a latticelike framework, and the actual decompositions arising as Boolean subalgebras. Central to the framework is the notion of bidimensional join dependency, which generalizes the classical notion of join dependency by allowing the components of the join to be selected horizontally as well as vertically. Several properties of such dependencies are presented, including a generalization of many of the classical results known to be equivalent to schema acyclicity. Finally, a characterization of the nature of dependencies which participate in decompositions is presented. It is shown that there are two major types, the bidimensional join dependencies, which are tuple generating and allow tuple removal by implicit encoding of knowledge, and splitting dependencies, which simply partition the database
Maintaining Consistency of Imprecise Relations
 The Computer Journal
, 1996
"... We extend functional dependencies (FDs), which are the most fundamental integrity constraints that arise in practice in relational databases, to be satisfied in an imprecise relation. The problem we tackle is the following: given an imprecise relation r over a relation schema R and a set of FDs F ov ..."
Abstract

Cited by 5 (0 self)
 Add to MetaCart
(Show Context)
We extend functional dependencies (FDs), which are the most fundamental integrity constraints that arise in practice in relational databases, to be satisfied in an imprecise relation. The problem we tackle is the following: given an imprecise relation r over a relation schema R and a set of FDs F over R, what is the most precise approximation of r, which is also consistent with respect to F. We formalise the notion of an imprecise relation by defining tuple values to be sets of values rather than just single values as is the case when the information is precise. We interpret each value in such a set as being equally likely to be the true value. This gives rise to equivalence classes of equally likely values thus allowing us to define the merge of an imprecise relation r which replaces values in r by their equivalence class. We also define a partial order on merged imprecise relations leading to the notion of an imprecise relation being less precise than another imprecise relation. Thi...
The Nested Universal Relation Data Model
"... this paper we propose to alleviate the usability problem by providing logical data independence to the nested relational model. To this end we extend the (classical) UR model to nested relations by defining the nested universal relation model (nested UR model). In particular, we extend the weak inst ..."
Abstract

Cited by 2 (0 self)
 Add to MetaCart
this paper we propose to alleviate the usability problem by providing logical data independence to the nested relational model. To this end we extend the (classical) UR model to nested relations by defining the nested universal relation model (nested UR model). In particular, we extend the weak instance approach to the UR model to the nested weak instance approach to the nested UR model. EXAMPLE 1.1. Schemas of nested relations are represented graphically by scheme trees [17], such as T shown in Fig. 1.1. The nested relation scheme (NRS) of T, denoted by R(T), is: 6 AIRLINE AIR_CODE (FLIGHT_NO (PASSENGER)* (CREW)*)* (AIRPORT PORT_CODE)*, where the higher order attributes are marked with * in order to distinguish them from the zero order attributes [1]. ################## ################## ################## # # # # ################## # # # # ################# ################# ############################### ############################### # # # # ############################### # # # # ############################### # # # # AIR_CODE AIRLINE CREW PASSENGER FLIGHT_NO AIRPORT PORT_CODE FIG. 1.1. The scheme tree T. A null extended nested relation (abbreviated to nested relation), r*, over the NRS, R(T), for the scheme tree, T, of Fig. 1.1, is shown in Fig. 1.2. We note that null denotes a generic null value, whose semantics are represented by the partial ordering, null is less informative than any nonnull value [17]. The semantics of r* can be expressed by a set of null functional dependencies (NFDs) and a set of null extended functional dependencies (NEFDs), both of which are members of the class of null extended data dependencies [17]. The NFDs that are satisfied in r* are: AIRLINE AIR_CODE, AIR_CODE AIRLINE, AIRPORT PORT_CODE and PORT_CODE AIRPORT. That is,...
Constanttimemaintainable BCNF Database Schemes
 ACM Transactions on Database Systems
, 1991
"... The maintenance problem (for database states) of a database scheme R with respect to a set of functional dependencies F is the following decision problem: Let r be a consistent state of R with respect to F and assume we insert a tuple t into r p 2 r . Is r [ ftg a consistent state of R with respect ..."
Abstract

Cited by 1 (0 self)
 Add to MetaCart
The maintenance problem (for database states) of a database scheme R with respect to a set of functional dependencies F is the following decision problem: Let r be a consistent state of R with respect to F and assume we insert a tuple t into r p 2 r . Is r [ ftg a consistent state of R with respect to F? R is said to be constanttimemaintainable with respect to F if there is an algorithm that solves the maintenance problem of R with respect to F in time independent of the state size. A characterization of constanttimemaintainability for the class of BCNF database schemes is given. An efficient algorithm that tests this characterization is shown, as well as an algorithm for solving the maintenance problem in time independent of the state size. It is also shown that total projections of the representative instance can be computed via unions of projections of sequential extension joins. Throughout we assume that database schemes are dependency preserving and BCNF, and that functional dependencies are given in the form of key dependencies. 1