uni/year2/semester1/logseq-stuff/pages/The Relational Model.md

• #CT230 - Database Systems I
• Previous Topic: Database System Introduction
• Next Topic: Introduction to SQL & DDL
• Relevant Slides:
• Why learn about relational DBMS?
  • 90% of industry / enterprise / business applications are still relational DBMS or relational DBMS with extensions (e.g., OO Relational).
  • The majority of industry applications require:
    • Correctness
    • Completeness
    • Efficiency (Complex optimisation techniques & complex indexing structures).
  • Relational DBMS provide this.
• What is the Relational Data Model? #card card-last-interval:: 21.53 card-repeats:: 4 card-ease-factor:: 2.32 card-next-schedule:: 2022-12-06T08:01:57.584Z card-last-reviewed:: 2022-11-14T20:01:57.584Z card-last-score:: 3
  • The Relational Data Model consists of collections of relations (often called tables) where each relation contains tuples (rows) and attributes (columns).
  • The relational data model is closely related to the file system model.
  • Relations are named.
• What is a relation? #card card-last-interval:: 31.36 card-repeats:: 4 card-ease-factor:: 2.8 card-next-schedule:: 2022-11-20T16:24:50.257Z card-last-reviewed:: 2022-10-20T08:24:50.257Z card-last-score:: 5
  • A table.
• What are attributes? #card card-last-interval:: 100.92 card-repeats:: 5 card-ease-factor:: 2.9 card-next-schedule:: 2023-02-23T18:22:23.155Z card-last-reviewed:: 2022-11-14T20:22:23.156Z card-last-score:: 5
  • Attributes are columns.
  • Columns / attributes are ^^almost always fixed^^ and do not change.
• What are tuples? #card card-last-interval:: 29.26 card-repeats:: 4 card-ease-factor:: 2.66 card-next-schedule:: 2022-12-13T22:48:34.835Z card-last-reviewed:: 2022-11-14T16:48:34.835Z card-last-score:: 5
  • Tuples are rows.
  • Rows contain the data.
  • There is a variable number of rows.
• What is the cardinality of a relation? #card card-last-interval:: -1 card-repeats:: 1 card-ease-factor:: 2.8 card-next-schedule:: 2022-11-15T00:00:00.000Z card-last-reviewed:: 2022-11-14T16:51:27.460Z card-last-score:: 1
  • The ^^number of tuples in a relation^^ is referred to as the cardinality of that relation.

• Attributes / Columns

  • Each attribute belongs to one domain and has a single:
    • Name
    • Data Type
    • Format

  • Naming Columns #card

    card-last-interval:: 28.3 card-repeats:: 4 card-ease-factor:: 2.66 card-next-schedule:: 2022-11-17T15:34:15.662Z card-last-reviewed:: 2022-10-20T08:34:15.663Z card-last-score:: 5
    • Case is not significant in SQL.
    • No spaces allowed.
    • No reserved keywords (e.g., date) allowed.
    • Choose meaningful variable names.
    • If given the names of relations and attributes, use ^^exactly^^ what you are given.

  • Data Types

    • You must ^^specify the data type^^ of all attributes (columns) defined.
    • Common data types used include:
      • varchar(N), where N is an integer - used for strings.
      • date
      • int
      • double
    • You often must specify the size - especially for integers and strings
    • 

    • NULL

      • What are null-valued attributes? #card card-last-interval:: 29.26 card-repeats:: 4 card-ease-factor:: 2.66 card-next-schedule:: 2022-12-13T22:48:31.350Z card-last-reviewed:: 2022-11-14T16:48:31.350Z card-last-score:: 5
        • Null-valued attributes are what occurs when the values of some attribute within a particular tuple may be unknown or may not apply to that particular tuple. A null value is used for these cases.
        • NULL is a special marker used in SQL to denote the ^^absence of a value.^^
      • In some cases, we wish to allow the possibility of a NULL value although they will often require extra handling (e.g., checking if == NULL).
      • In other cases, we want to prevent NULL from being entered as a value and specify NOT NULL as a constraint on data entry.

  • Atomic Attributes

    • What is an atomic attribute? #card card-last-interval:: 33.64 card-repeats:: 4 card-ease-factor:: 2.9 card-next-schedule:: 2022-12-18T07:51:31.019Z card-last-reviewed:: 2022-11-14T16:51:31.019Z card-last-score:: 5
      • An atomic attribute is an attribute which contains a ^^single value of the appropriate type^^, generally meaning, "no repeating values of the same type".
      • The relational model should only have atomic values.

  • Composite Attributes

    • What is a composite attribute? #card card-last-interval:: 33.64 card-repeats:: 4 card-ease-factor:: 2.9 card-next-schedule:: 2022-12-18T07:52:00.872Z card-last-reviewed:: 2022-11-14T16:52:00.872Z card-last-score:: 5
      • A composite attribute is an attribute that is composed of several atomic attributes.
        • E.g., Name = FirstName, Middle Initial, Surname.
      • We often want to decompose a composite attribute into atomic attributes unless there is a very good reason not to.

  • Multi-Valued Attributes

    • What is a multi-valued attribute? #card card-last-interval:: 4.14 card-repeats:: 2 card-ease-factor:: 2.56 card-next-schedule:: 2022-11-26T16:37:36.766Z card-last-reviewed:: 2022-11-22T13:37:36.767Z card-last-score:: 5
      • A multi-valued attribute is an attribute which has lower and upper bounds on the number of values for an individual entry.
        • The ^^opposite of an atomic attribute.^^
      • The relational model should not store multi-valued attributes.
        • Database design / redesign should be used to deal with this issue by creating more attributes (columns) or more tables.

    • Derived Attributes

      • What are derived attributes? #card card-last-interval:: 28.3 card-repeats:: 4 card-ease-factor:: 2.66 card-next-schedule:: 2022-12-12T23:52:22.646Z card-last-reviewed:: 2022-11-14T16:52:22.646Z card-last-score:: 3
        • A derived attribute is an attribute whose value can be determined from another attribute.
          • E.g., you can derive age from birthdate.
        • It is a good idea to not directly store attribute which can be derived from other attributes.

• Collection of Relations

  • A Relational Data Model consists of a collection of relations (tables).
    • Tables are cross-linked.
• • A relational database usually contains many relations (tables) rather than storing all data in one single relation.
  • What is a Relational Schema? #card card-last-interval:: 19.3 card-repeats:: 4 card-ease-factor:: 2.32 card-next-schedule:: 2022-12-04T03:03:56.818Z card-last-reviewed:: 2022-11-14T20:03:56.818Z card-last-score:: 3
    • A relational schema, R, is the ^^definition of a table in the database.^^ It can be denoted by listing the table name and the attributes:
      • R=\{A_1,A_2,...,A_n\}
        • where A_i is an attribute.
      • E.g., with n=3, works_on(essn, pno, hours).

• Linking Tables

  • Two ^^extremely important concepts^^ within the relational model which allows tables to be linked & cross-referenced are:
    • Primary Key attributes.
    • Foreign Key attributes.

  • Primary Keys

    • Fundamental concept of Primary Keys: #card card-last-interval:: 28.3 card-repeats:: 4 card-ease-factor:: 2.66 card-next-schedule:: 2022-11-17T15:34:03.291Z card-last-reviewed:: 2022-10-20T08:34:03.291Z card-last-score:: 5
      • ^^All tuples (rows) in a relation must be distinct.^^
      • To ensure this, we must have one or more attributes / columns whose data values will ^^always be unique for each tuple^^ - these attributes are called key attributes and are used to identify a tuple in the relation.
      • There may be a few possibilities for the primary key - these are called Candidate Keys.
        • One candidate key is ultimately chosen as the primary key during the Design Stage.
    • What is a Primary Key? #card card-last-interval:: 23.43 card-repeats:: 4 card-ease-factor:: 2.42 card-next-schedule:: 2022-12-08T02:44:46.959Z card-last-reviewed:: 2022-11-14T16:44:46.960Z card-last-score:: 5
      • A primary key is defined as one or more attributes per table where:
        • there can only be one such primary key per table
        • the primary key can never contain the NULL value
        • all values entered for the primary key must be unique (no duplicates across the rows)
      • Often, primary keys are used as indices.
      • We use the convention (in writing) that attribute which form primary keys are \text{\underline{underlined}}.

  • Foreign Keys

    • What is a Foreign Key? #card card-last-interval:: 23.43 card-repeats:: 4 card-ease-factor:: 2.42 card-next-schedule:: 2022-12-08T02:52:18.585Z card-last-reviewed:: 2022-11-14T16:52:18.585Z card-last-score:: 3
      • A Foreign Key is an attribute, or a set of attributes, within one table that matches or links to a candidate key of some other table (possibly the same table).
      • More formally:
        • Given relations r_1 and r_2, a foreign key of r_2 is an attribute (or set of attributes) in r_2 where that attribute is a candidate key in r_1. Relations r_1 and r_2 may be the same relations.

    • Foreign Key Terminology

      • The parent, master, or referenced table is the relation containing the candidate key(s).
      • The child or referencing table / relation is the relation containing the foreign key.