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Limitations of the relational model. Limitations of the relational model. Just as the relational model supplanted the network and hierarchical model so too will the object – orientated model supplant the relation model ? Is this true? Why?. Major strengths of the relational model.

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limitations of the relational model2
Limitations of the relational model

Just as the relational model supplanted the network and hierarchical model so too will the object – orientated model supplant the relation model ?

  • Is this true?
  • Why?
major strengths of the relational model
Major strengths of the relational model
  • Data model and access is simple to understand and use
  • Access to data via the model does not require navigation (following pointers) as the network models
  • Allows (in principle) declarative query language
  • There are straightforward DB design procedures
  • Admits a solid and well understood mathematical foundation (RA)
  • Implementation techniques are well known, efficient and widely used.
slide4
Cont…..
  • Standards exist both for query languages (SQL) and for interfaces via programming languages (embedded SQL, ODBC)
  • Why expand to another model?
why go beyond the r model
Why go beyond the R model
  • Some forms of data and knowledge which cannot be accommodated easily and adequately
  • OO programming languages are emerging as the dominant form within development environments for large-scale software systems.
  • Language independent system environments which are based upon OO models are emerging and may be extremely important in the future
    • Object management group (OMG) standards
limitation 1
Limitation 1
  • Some forms of data and knowledge which cannot be accommodated easily and adequately
  • There are always very special types of data which require special forms of representation
    • Temporal data, Spatial data, Multimedia data, unstructured data (WH, DM),Documents libraries (digital libraries)
  • Limitations regarding SQL3 as the query language
    • Recursive queries
object identify
Object identify
  • ER modelling, object types such as employee, department, project etc are specified in the R model these survive only as names of relations/tables
  • In R model entities have no independent identification of existence, objects are identified and access via identification of attributes characterising them
    • E.g.: a company db will have an entity of employee, yet in the R model the employee exists by virtue of a list of attributes in some tables.
explicit relationships
Explicit relationships
  • ER modelling explicit entities and relationships were specified in the R model the identities of relations have no explicit relations
  • Relationships must be known to the user
  • There are hidden semantics in the R model

Supervisor

Works on

employee

supervision

employee

project

Supervisee

ER

R

structured data objects
Structured data objects
  • 1NF stipulates that the values for attributes in a row be atomic
  • Prevents complex values below in which the values of the domains are themselves rows
    • No collection types

Name NI DoB Addres Gender Salary Empno

Fname initail Sname

No street town city PC

generalisation and inheritance
Generalisation and inheritance
  • Classes of entities to be modelling a db often have natural hierarchical structure

generalisation

Class of objects associated with a type higher is a superset of that association

Every employee is a teacher

Every student instructor is both a student and an instructor

Classes below inherit attributes from those above

Inheritance not in R model

person

student

employee

teacher

specialisation

Student

instructor

methods
methods
  • Often it is convenient to record explicitly special queries on a database
  • R model for read only queries this is accomplished via views
    • Cost overhead and need system to maintain the current value
  • R model of updates has no similar mechanism procedures must be maintained outside of the model itself.
    • E.g. add employee
strategies for addressing issues
Strategies for addressing issues
  • 2 main philosophies
  • Extend the R model to accommodate features to overcome these short comings
  • Start for scratch
    • It is not feasible to extend the R model in this way
  • Both approaches have been tried over the past 20 years.
extensions to the r model
Extensions to the R model
  • A number of vendors have added special features to their R database
    • Constraint! Any extension must be compatible with the SQL2(SQL-92) standard Oracle, IBM, HP, Informix/Ilustra/Mico, UniSQL
  • Although futures may be similar they are not compatible beyone the SQL2 level
  • In addition there have been attempts to extend SQL to accommodate desired features
slide14
Cont…
  • SQL1999 (AKA SQL3) a standard which has recently been completed and addresses some of the concerns
  • SQL4 a standard currently under development to address other issues
  • These standards attempt to be compatible with earlier versions of SQL with small changes.
fundamentally oo systems
Fundamentally OO systems
  • During the past 20 years a number of OO db systems have been developed, these largely abandon the R model and start from scratch with an OO db key examples are:
    • O2, GemStone, OjectStore
  • Each system displayed strengths for certain types of application
  • System are not compatible with each other
    • Lead to need for standards for next generation of systems ODMG (object database management group)
overall summary of current directions
Overall summary of current directions
  • Bring databases ideas into the existing OO world
    • The database model is inherently OO; relational ideas are abandoned
    • There is no stand-alone query language
    • Access to the DB requires a host of OO programming language
    • Emerging standard: ODMG proposals
slide17
Cont …
  • Bring OO concepts into the existing relational database world
    • The R model is extended to admit certain OO ideas
    • Access is via an extended version of SQL
    • Access via queries embedded in programming languages is also possible
    • Emerging standards SQL3, SQL4
slide18
Cont …
  • Develop a general framework for manipulating objects in a interoperable environment
    • Framework is not specific to DBMS
    • Deals with general object services in a distributed heterogeneous environment
    • Existing standards OMG (object management group) proposals
bottom line which is best
Bottom line – which is best?
  • Relational model v Object relational model V OO model
  • Depends on the application at hand
  • No one of these is superior to the others in all possible situations
  • A better understanding of these approaches can help to decide which is most appropriate for a given application
summary of the efficiency of these approaches
Summary of the efficiency of these approaches
  • SQL3/SQL4
    • Extensions do add some needed features but definitions seem to be ad hoc and not based upon sound OO theory
  • ODMG proposal
    • Foundations more solid to OO systems but advantages of R model lost
      • Needs expertise to use systems
      • Schema design is much more involved
      • In many cases the system are orientated to wards a specific OO host language.
  • OMG framework
    • Already becoming standard details of which are outside the scope of the module.