2 relational database

Outline: A single data structure: Relationships

Two, relational operation: relational algebra language; relational calculus language; SQL

III. integrity constraints: Entities, references, user customizations

I. Integrity constraints

1. Entity integrity: The individual components of the main attribute cannot be null values

2. Referential integrity: The outer code is either empty or equal to the primary code value in the referenced relational table

3. User-defined Integrity

Second, relational algebra

A) Traditional set operations:

1. and

2. and

3. Poor

4. Generalized Cartesian product

(ii) Specialized relational operations

1. Select σ: The tuple of the approved conditions in the relationship R; σsage<20 (Student) selects sage<20 rows from Student

2. Projection π: Select several columns from the relationship R as a new relationship πsname,sdept (Student) Choose sname columns and sdept columns from Student as a new table

3. Connect ∞

1) General Connection

2) Equivalent connection

3) Natural Connection: You need to remove duplicates and speak the same part of the two groups

4. Difference ÷

2 relational database