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
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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
Database principle and application--relational database