1 / 9

Relational Model

Relational Model. Database Management Systems, 3 rd ed., Ramakrishnan and Gehrke, Chapter 3. Relation.

genica
Télécharger la présentation

Relational Model

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Relational Model Database Management Systems, 3rd ed., Ramakrishnan and Gehrke, Chapter 3

  2. Relation Given a collection of (not necessarily distinct) sets D1, D2, …, Dn, r(R) is a relation on those n sets if it is a set of ordered n-tuples <d1, d2, …, dn> such that d1 D1, d2 D2, …, and dn Dn. Sets D1, D2, …, Dn are the domains of r(R). n is the degree or arity of r(R). Formal defns. taken from Fundamentals of Database Systems by Elmasri and Navathe

  3. Relation Schema A relation schema or intension R, denoted by R (A1, A2, …, A), is a set of attributes R = {A1, A2, …, An}. Example of relation schema: Student (ID: string, Name: string, Age: integer, Gender: char) Relation r of the relation schema R, denoted r(R), is an instance or extension of the schema. E.g.: <2739, Peter Wong, 20, M>

  4. Relationship to Predicate Logic Codd based relational model, in part, on predicate logic. Think of relation schema as a predicate. Example: Father (NameOfSon, NameOfFather) <John Doe, Joe Doe> <Joe Doe, Bob Doe> Rule: If x is the father of y, and y is the father of z, then x is the grandfather of z.

  5. Relationship to Set Theory • No duplicate tuples (although allowed by most commercial DBMSs) • Significance of ordering of attributes in tuples (although generally ignored by relational languages except when inserting new tuples) • No significance in ordering of tuples (unless sorting specified)

  6. Keys K is a candidate key of relation R if it is a collection of attributes of R such that, independent of time, the following properties hold: • No two tuples of R have the same value of K. • If any attribute of K is dropped, then the uniqueness property is lost. Must select a CK to serve as the primary key. A superkey is a collection of attributes for which a subset of the attributes is a candidate key.

  7. Database Schema A relational database schema S is a set of relation schemas S = {R1, R2, …, Rn} and a set of integrity constraints. IC – condition specified on d.b. schema that restricts data that can be stored in instance of d.b. A relational database instance DB of S is a set of relation instances DB = {r1, r2, …, rm} such that each ri is an instance of Ri and the ris satisfy the ICs.

  8. Constraints Key constraints specify the CKs of each relation schema. A domain constraint specifies that the values that appear in a given column must be drawn from the domain for that column. The entity integrity constraint states that no PK can be null. The referential integrity constraint specifies that a tuple in one relation that refers to another relation must refer to an existing tuple in that relation.

  9. Foreign Key A set of attributes FK in relation schema R1 is a foreign key of R1 if it satisfies the following: • The attributes in FK have the same domain as the primary key attributes PK of another relation schema R2; the attributes FK are said to reference or refer to the relation R2. • A value of FK in a tuple t1 of R1 either occurs as a value of PK for some tuple t2 in R2 or is null.

More Related