Database principle and application--relational database

Relational data Structures

Entities, relationships between entities are relational representations, and the logical structure of the user's angle is the two-dimensional table

Relationship: A subset of Cartesian product

Relationship Operations Collection

• Traditional set operation: And, intersection, difference, Cartesian product

• dedicated relational operations: Select , projection , connection ( Connection, equivalent connection, natural connection)

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  for s projection y, Y for R, and for other items, see two inclusive relationships

• Extended operation of relational algebra

  Outer connection (left outer connection, right outer connection, full outer connection)

  Left OUTER join: Removes all tuples in the left relationship that do not match any tuple in the right relationship, with the ? Replace right-side relationship property

Relationship Integrity Constraints

• Entity integrity: Primary key is not ?

• Referential integrity: Foreign keys do not allow referencing entities that do not exist

• User-defined Integrity

Database principle and application--relational database