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Relational Model in DBMS

History of Relational Model

Proposed by E.F. Codd in 1970
  • "A relational model of data for large shared data banks"
Objectives of relational model
  • High degree of data independence
  • Dealing with issues e.g. data semantics, consistencies, and redundancy
  • To enable the expansion of set-oriented data manipulation languages.
Examples of RDBMSs
  • Oracle
  • Microsoft SQL Server
  • Microsoft Access
  • Visual FoxPro
  • InterBase
  • JDataStore
  • R:Base
Relational Model
  • Based on mathematical concept of a relation.
  • Codd, a trained mathematician, used terminology taken from mathematics, principally set theory and predicate logic.
Relational Model Terminology
  • A relation is a table with columns and rows
  • Attribute is a named column of a relation
  • Domain is the set of allowable values for one or more attributes.
  • Tuple is a row of a relation
  • Degree is the number of attributes in a relation
  • Cardinality is the number of tuples in a relation
  • Relation Database is a collection of normalized relations with distinct relation names.
Instances of Sample Relation

Examples of Attribute Domains

Alternative Terminology

Mathematical Relations

  • Consider two sets, D1 & D2
             - Where D1 = {2,4}  and D2 = {1,3,5}
  • Cartesian product, D1 х D2 is,
            - Set of all ordered pairs where first element is member of D1 and second element is member of D2 
                   * D1 х D2 = { (2,1) . (2,3) . (2,5) . (4,1) . (4,3) . (4,5)
                   * Alternative way is to find all combinations of elements with first from D1 and second from D2.

Any subset of Cartesian product is a relation; e.g.
  • R = { (2,1), (4,1)}
  • May specify which pairs are in relation using some condition for selection; e.g.
  • Second element is 1:
             - R = {(x,y) | x є D1, y є D2, and y = 1}
  • First element is always twice the second:
            - S = {(x,y) |  x є D1, y є D2, and x =2y}
  • Relating with database relations
        - Consider Branch relation in the example where branchNo = {B002, B003}  city = {London, Glasgow}
  • Cartesian product, branchNo х city is,
         - {(B002, London), (B002, Glasgow), (B003, London), (B003, Glasgow)}
  • Any subset of Cartesian product is a relation

Database Relations

Relation schema
  • Named relation defined by a set of attribute and domain name pairs
  • Let A1, A2,....,An be attributes with domain D1, D2,....,Dn
  • Then the set {A1:D1, A2:D2, ..... , An:Dn} is a relation schema
  • A relation T defined by a relation schema S is a set of mapping from the attributes names to their corresponding domains. Thus, relation R is a set of n-tuples:
  • (A1:d1, A2:d2,....., An:dn) such that
  • d1∈D1, d2∈D2,.....,dn∈Dn
  • Branch relation schema
         * Branch = { branchNo:BranchNumbers, street:StreetNames, city:CityNames, postcode:PostCodes}
  • Tuple of a Branch relation
         {branchNo:B005, street:22DeerRd, city:London, postcode:SW1 4EH}

Relational Database Schema
  • If R1, R2,..., Rn are a set of relation schemas, then we can write the relational database schema, or simply relational schema, R, as:
           R = {R1, R2,....., Rn}

Properties of Relations
  • Relation name is distinct from all other relation names in relational schema.
  • Each cell of relation contains exactly one atomic (single) value
  • Each attributes has a distinct name
  • Values of an attribute are all from the same domain
  • Each tuple is distinct; there are no duplicate tuples
  • Order of attributes has no significance
  • Order of tuples has no significance, theoretically

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