Database technology-relational database

• Relational database structure

  • Relationships : a single data structure, real-world entities and various connections between entities

  • Two-dimensional table : From the user's perspective, the logical structure of the data in the relational model is a two-dimensional table

  • Domain : a set of values ​​with the same data type

  • Relationship representation : each row represents a tuple, and each column corresponds to a field

  • Attribute : the name of each column

  • Code :

    • Candidate code : The value of an attribute group in the relationship can uniquely identify a tuple, then the attribute is called a candidate code
    • Full code : In the most extreme case, all attribute groups in the relationship are candidate codes for this relationship mode
    • Master code : select a master code from multiple candidate codes
    • Main attribute : The attribute of the candidate code is called the main attribute

  • nature

    • Different columns can be the same domain
    • The order of the columns does not matter
    • Candidate codes for any two tuples cannot be the same
    • The order of the rows does not matter
    • Amount must take atomic value

  • Relationship model

    • The description of the relationship is static
    • R (A1, A2, A3, A4 ...), R is the relationship name, A1, A2, A3 ... is the attribute name
    • The value of the relationship: the state or content of the relationship pattern at a certain moment

  • relational database

    • Definition: For a given application, the collection of all relationships constitutes a relational database
    • Type: description of relational database
    • Value: A collection of relational patterns defined on these fields

  • Relational operation

    • Query, select, project, connect, intersect, merge, difference
    • Data update: insert, delete, modify

  • Relational algebra

    • An abstract query language for relational databases, expressing queries with relational operations

• Integrity of relationship

  • Integrity constraints

    • Integrity constraints that the relational model must satisfy: entity integrity and referential integrity—two invariants
    • Constraints to be followed in application areas

  • Entity integrity rules

    • A basic table usually corresponds to a set of entities in the real world
    • Real-world entities are distinguishable and have unique identification
    • The main code is used as the unique identifier in the relationship model
    • The main attribute (main code) cannot be null

  • Referential integrity

    • References between relationships: references to different attributes

    • Foreign code:

      • F is an attribute in the relationship R, but not the main code, and F corresponds to the main code of the relationship S, then F is the outer code of R

    • Referential integrity rules

  • User-defined integrity

    • Constraints for a specific relational database

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