Chapter II relational database after class exercises

1. Try to describe the three components of the relational model

 The relational model consists of relational data structure 、 Set of relational operations 、 The integrity constraint of the relational model consists of three parts ;

2. Try to describe relational data ⾔ Characteristics and classification of .

 Relational data languages can be divided into three categories ：

1 Relational algebra language ;
2 The language of relational calculus ;
3 SQL： A language with the dual characteristics of relational algebra and relational calculus ;
The common characteristics are ： Language has a complete ability of expression , It is the operation language of non procedural set , Powerful function ⼊⾼ Level language ⾔ Chinese envoy ⽤

3. Give an example to illustrate the difference between relationship mode and relationship .

 Relationship is the state and content of a relationship pattern at a certain time ; The relational pattern can be said to be static 、 The stability of the ; Relationships are dynamic 、 Changing over time ; Relationship is the description of relationship pattern ;

4. Try to explain the integrity rules of relational patterns . In referential integrity , Under what circumstances can the value of foreign code attribute be Null value ？

 If the attribute is not the primary attribute, it can be null , Otherwise, we can't .

5. Try to describe equivalent connection and ⾃ However, the difference and connection of connection .

 The join operator is “=” The connection operation of is called equivalent connection . It's from the relationship R And S Of ⼴ Select from the Cartesian product A、B Those tuples with equal property values 
 Natural connection can be said to be a special equivalent connection , It is required that the components to be compared in the two relationships must be the same attribute group ; Also remove the duplicate attribute columns .

6. What are the basic operations of relational algebra ？ how ⽤ These basic operations represent other operations ？

 and 、 Bad 、 The cartesian product 、 Projection and selection 5 This operation is the basic operation . other 3 Operations , Hand in immediately 、 Connect and remove , All are OK ⽤ this 5 There are three basic operations to express .

Mind mapping ：