1.3. Joins

In this lecture we look at...
[Section notes PDF 233Kb].

1.3.01. Introduction

  • Recap: pulling data out of individual relations
    • By row, by column
    • Select and project
  • Access across multiple relations
  • Miniworld approximation
    • Fragmenting entities by cardinality
    • Tuples as entity fragments
    • Relationships within relations
  • Joins
  • Join types (condition and unmatched)

1.3.02. Access across relations

  • Relational model allows multiple relations to exist within one database schema
  • Relations can be accessed individually or together (joins).
  • Referential integrity
    • Relations relating
  • Pulling data out of single relations
    • Select and project
  • Pulling related data out of
    • Multiple relations using Join

1.3.03. Miniworld approximation

  • Universe of Discourse, or Miniworld
  • Miniworld is an incomplete model of the real world
  • The relational data model as a model for the miniworld
  • Approximation
    • Separate and distinct entities
    • Single complex entities
    • Separate related entities
    • Cardinality of relationships
  • Each relation made up of attributes
  • Values can be used as references

1.3.04. Pointing mechanism

  • Relation has a Primary key
  • Tuple contains Primary key value
  • Foreign keys
    • Tuples can contain a reference to another relation's Primary key
  • Just numbers

Relational model foreign key example

One number identifies a single tuple in one relation (local), one number identifies a single tuple in another relation (foreign).

1.3.04b. Pointing mechanism example in C

  • C programming language
  • Memory addresses, or pointers
    int a=0;
    int b=0;
    a = &b;
  • a points to b

relational foreign key C programming example

In databases, typically done with unique identifiers (IDs) rather than memory addresses.

1.3.04c. Pointing mechanism with structures

  • Foreign key importing
    typedef struct car
      int ID;
      char[] make;
      char[] model;
      char[] derivative;
      int optionID;
    } car;
    typedef struct option
      int ID;
      char[] name;
      int price;
    } option;
    car c;
    option o;
    //...data structure populating
    c.optionID = o.ID;

Relational foreign key example in C with Structures

1.3.05. Relational cardinality

  • 1:0 relationships
    • Single entity
    • Uniquely indentifiable
    • Candidate keys
    • Primary Key
  • 1:1 relationships
    • Two entities, A and B
    • 1 A relates to 1 B and vice versa
  • 1:N relationships
  • M:N relationships

1.3.06. Relationships in the relational model

  • Two relations, A and B
  • A side, B side, 1 side, N side
  • 1:1 relationships
    • Key can go on either side

Relational model 1 to 1 relationships

  • 1:N relationships
  • Key cannot go on 1 side
  • Has to go on N side

Relational model 1 to N relationships

  • M:N relationships
    • Nowhere obvious for the key to go
    • Create new pairing relation

1.3.07. Joins

  • Phase change, different point in lifecycle
  • Join operation
    • Combines related tuples, conditionally
    • From two relations
    • Into single tuples
  • Allows processing of relationships
  • Among multiple relations

1.3.08. Joins, canonical algebraic form

  • Conditional (on join condition)
    • Only combines tuples where true
  • Cartesian product (conditionless)
    • example of conditionless join
    • all tuples combined
    • R ⋈true S
  • ⋈, Binary operator
    e.g. R ⋈<join_condition> S

1.3.09. Join equivalence

  • Equivalent to sequence
    • Cartesian product (X)
    • followed by Selection (s)
  • or

1.3.10. Join types (condition)

  • Theta: Ai q Bj
    (A from R, B from S)
    • q is comparison operator
    • Ai and Bj share the same domain
  • Equi: Ai = Bj
    • Theta join where q is =
  • Natural: Ai and Bj are the same attribute
    • in two separate relations (name and domain)
    • * denotes natural join

Relational join example, equijoin

1.3.11. Join types (inner and outer)

  • Inner joins
    • not the only joins
    • eliminate tuples without a matching counterpart
    • i.e. tuples with a null value for the join attribute are discarded

Relational Inner join demonstration

1.3.12. Outer joins

  • Outer joins control what's discarded
    • Keep unmatched tuples in either
      • Left, right, or both relations
      • Left, right of full outer join correspondingly

    Relational demonstration of Outer joins